Atomic Heart

There are a total of 50 corpses in the game, 41 in story locations that are skippable and 9 in the open world.
I can confirm that the achievement is doable. The second time I abandoned the passage after I noticed that some corpses did not accrue progress. Today I tried to pass for the third time and for a very long time the achievement progress was not accrued, but then I immediately received more than 20%, and after that each new corpse accrued 2%. I used this guide to search. I can confirm that those corpses that raise doubts about the crediting of progress indicated in this guide do not really credit the progress, but this does not prevent you from getting the achievement.
Facility 3826

Forester Area

VDNH

Open World

Lastochkin Theater

Pavlov

There are a total of 50 corpses in the game, 41 in story locations that are skippable and 9 in the open world.
I can confirm that the achievement is doable. The second time I abandoned the passage after I noticed that some corpses did not accrue progress. Today I tried to pass for the third time and for a very long time the achievement progress was not accrued, but then I immediately received more than 20%, and after that each new corpse accrued 2%. I used this guide to search. I can confirm that those corpses that raise doubts about the crediting of progress indicated in this guide do not really credit the progress, but this does not prevent you from getting the achievement.
Facility 3826

Forester Area

VDNH

Open World

Lastochkin Theater

Pavlov

We know that this achievement is broken, although it can only be that the tracker does not work, in any case I will be making guides with all the collectibles and I will tell you if collecting them all unlocks the achievement. There are some talking dead that you can't interact with, I don't know if they really count towards the achievement or if they are just a bug but I included them in the guide in case they fix this so I recommend having save points in each area of ​​the game.

Vavilov Complex and Forester Village:

VDNH Complex And Plisetskaya Theater:

Started a new game yesterday 27/02/23 to see if I could record some footage for a guide on this. Unfortunately the tracking is still an issue and multiple Bodies couldn't be spoken to or the tracking didn't progress. What a joke!

Missable: If you leave a mission location such as Vavilov, VDNH, Theater or Pavlov having missed a Dead Body there's no way of returning to these areas to talk to them.

There's around 50 Dead Bodies in the game to talk to, although documented online it's still broken. Due to the issue with achievement tracking resetting to 0% this looks nearly impossible to do in its current state. You'd have to find all bodies in one sitting without closing down the game.

The people that currently have it unlocked like myself, managed to save glitch it until the recent patch stopped that.

Marking this one as broken until it's patched.

Started a new game yesterday 27/02/23 to see if I could record some footage for a guide on this. Unfortunately the tracking is still an issue and multiple Bodies couldn't be spoken to or the tracking didn't progress. What a joke!

Missable: If you leave a mission location such as Vavilov, VDNH, Theater or Pavlov having missed a Dead Body there's no way of returning to these areas to talk to them.

There's around 50 Dead Bodies in the game to talk to, although documented online it's still broken. Due to the issue with achievement tracking resetting to 0% this looks nearly impossible to do in its current state. You'd have to find all bodies in one sitting without closing down the game.

The people that currently have it unlocked like myself, managed to save glitch it until the recent patch stopped that.

Marking this one as broken until it's patched.

*Played on the Series S, via Game Pass*

From my experience and numerous videos, I have concluded that there are 49 corpses that can be interacted with, 2 more are highlighted as if should be but alas you cannot interact with them so these won't contribute. Additionally, there are a total of 3 talking bodies whose interactions are slightly different from all others; there is no dialogue box for a response from P-3, and the corpse's title is not displayed. I have not been able to confirm, but I believe 1 or more of these bodies do not count in the tracker, but you might as well talk to them just to be safe. I didn't start watching the tracker until after the Theater mission, however, based on my rate of progress for each new corpse, I came up with a qty of 45 corpses needed for the achievement. While watching it, I didn't have issues with the achievement tracker, and my progress rate was in line with ~2.22% (=100/45) progress per talking corpse. The tracker did not account for the decimal, nor did it appear to round up, so taking those into consideration the math added up to suggest 45 was the lucky number. For example, 4 corpses would be 8.88% progress, but the tracker showed 8%. Therefore, every 9th one found = 20% progress.
1 2.22 = 2%
2 4.44 = 4%
3 6.66 = 6%
4 8.88 = 8%
5 11.11 = 11%

For me, the Achievement popped when interacting with "Evil Dead Man" in the Pavlov complex, which aligned with my progression tracker so I don't believe this was a fortunate glitch. This left another 2 corpses remaining in the Pavlov Center that were "Extra", so there should be some wiggle room if you happened to miss an earlier corpse.

There are plenty of videos available so I won't duplicate that here, but what I have done is added the list of discovered Talking Bodies that can be discovered according to the location aligned with story progress, primarily to avoid back and forth in the Open World area. Hope this helps.

Facility 3826 - Vavilov Complex
Chatty Corpse Mandatory - has Lock Code
Dead technician After cable car falls and Burva tunnels ahead of you
Scared Dead Man Inside of an upper floor in the Birchtree area
Trapped Man Heading to Algae Lab, after dealing wiht mutants, objective maker has you going left through a gated door, instead go right and near the dead end under some debris
Chatty Corpse Upper platform opposite the Algae Polymer Insert console
Chatty Corpse On the way to the Pesticide and old Workshop, where the cooridor splits, under a yellow and blue sign
Dead Scientist Room after the windows showcasing the "mother" then the Chicken, and finally the mutants that are accidentally alerted
Dead Florist After meeting the person sumping pesticide on the large plant, decend to the ground floor, this corpse is in the corner under some greenery blockign the path
Dead Romantic When guiding the pesticide container to the plant, you will use telekenesis to reach an area in the center, climb onto the damaged platform and over to the otherside of the room, the corpse is on the elevated railing near a chest in a dark corner.

Forester Area
Dead Loser End of the road, past the elevated train station
Dead Conductor Under the Train Station, behind a small shack near the stairs
Dead Soldier Across the street from the Train Station - has a train ticket
Dead Technician In the upper corner of the building up the street from the Train Station, use the polymer trail to swim into the open window - has a Train Pass

Open World
Dead Game Developer Back corner, opposite the stairs leading down, from the crashed train car when first entering the area
Fallen Corpse South West of the crashed Train car, near the area boundary past the lone building

Exhibition Center - VDNH
Dead Technician On the floor of the second magent puzzle room, back corner near the broken stairs
Dead Engineer On the floor of the third magenet puzzle room, back corner.
Dead Tour Guide First office on the left, opposite the pattern locked Tokamak Complex display room
*Talking Body Room with the Plant item for Tereshkova (FemBot)
Dead Tour Guide Opposite the entrance to the main VDNH facility on the uppr floor.
Suicidal Corpse Main VDNH facility, 2nd floor stair well near some black roots/tentacles.
Dead Tour Guide Left of the "moon" lock on the second floor, near the room's back doorway
Defiant Dead Man Through the doors opened by the previous corspe, at the top of the stairs to the left
Defiant Ichthyologist Pavilov Floor, near the area's chirper on the walkway overlooking the animal organ projection
Taciturn Corpse Past the Plyusch fight, in a lab with several circular operating stations and numerous Lab Tech Vova, its laying in one of operation station bays.
Cranky Talking Corpse Aquarium area, ground floor
Silly Corpse Near the satellite relay puzzle, requried for Claire's head
Drowned Man On the second floor of the Aquarium, climb the crashed Beluga craft to reach.

Open World
Glitched Dead Near the exit from VDNH Complex, the body is on the road but cannot be interacted with (doesn't count)
Dead Weird Scientist Next to the building on the upper area connected to the dock along the river near Testing Ground 9
Strange Corpse Small Crater in the grass near the main staircase (NW) leading up to the Kollective Complex in the Testing Ground 9 region.
Dead Accountant Bathroom of the Sechenov Reseach Centre lobby, where you go to confront Petrov and Filatova, near Testing Gorund 8
Dead Farmer Near the Testing Groung 10 entrance, among some hay bales, near the area's Volan (Hawk's relay station)
Stinky Corpse In the Testing Ground 10 lower region, far south east side past the 2 buildings away from the road, under a pile of manure
Dead Milkwoman Along the lower farm area's road between Testing Ground 10 and 2, just past a tall white wind turbine
Dead Bureaucrat Near Testing Ground 2, below the white platform with the spiral staircase leading to the Hawk Relay.

Lastochkin Theater
*Talking Body Right, after leaving the entrance elevator
*Talking Body Destroyed room on the third floor after powering up the elevator, there is a yellow ledge observed from Chirper location for "What a Performance" you can reach by jump dashing through the partially collapsed wall to a yellow marked floor; above the Mother/Sprout filled room.
Glitched Dead 2nd floor room, under the location from the previous corpse, body is highlighted but cannot interact with. (doesn't count)
Concession Stand Attendant Ballerina Performance Act 1: Space Odyssey area - Behind the concession on the lower level
Talking CorpseBallerina Performance Act 1: Space Odyssey area - 2nd floor, lying next to the pattern locked door leading tells you to find the electrician for the door code
Talking Corpse Electrician who holds the door's pattern code, story related and will be marked with a waypoint unless you enter the code already
Dead Prima *1 There are two bodies in the back dressing room where the all the mutants attack during the performance. I am assuming each corpse is individual and therefore both will contribute towards your progress, may need to confirm.
Dead Prima *2 see above
Lastochkin's Corpse After the Boss Battle, behind the stage in a corner

Infirmary
Talking Corpse 2nd floor of the Infirmary, head up the stairs from the main entrance and it is under debris in front of a blocked doorway
Paranoiac's Corpse Enter the building at street level, this will be in one of the back rooms on the floor near a bed.
Talking Corpse Another body on a gurney on one of the back rooms when you first enter the Infirmary building

Pavlov Complex
Evil Dead Man Pass through the Disk Locked door, in a dark corner behind a gurney with a body on display
Dead Florist 3rd Floor of the Morgue, next to the locked door exit
Dead Doctor Distribution Centre in the middle of the room filled with robots, at the feet of large gold statue.

*Played on the Series S, via Game Pass*

From my experience and numerous videos, I have concluded that there are 49 corpses that can be interacted with, 2 more are highlighted as if should be but alas you cannot interact with them so these won't contribute. Additionally, there are a total of 3 talking bodies whose interactions are slightly different from all others; there is no dialogue box for a response from P-3, and the corpse's title is not displayed. I have not been able to confirm, but I believe 1 or more of these bodies do not count in the tracker, but you might as well talk to them just to be safe. I didn't start watching the tracker until after the Theater mission, however, based on my rate of progress for each new corpse, I came up with a qty of 45 corpses needed for the achievement. While watching it, I didn't have issues with the achievement tracker, and my progress rate was in line with ~2.22% (=100/45) progress per talking corpse. The tracker did not account for the decimal, nor did it appear to round up, so taking those into consideration the math added up to suggest 45 was the lucky number. For example, 4 corpses would be 8.88% progress, but the tracker showed 8%. Therefore, every 9th one found = 20% progress.
1 2.22 = 2%
2 4.44 = 4%
3 6.66 = 6%
4 8.88 = 8%
5 11.11 = 11%

For me, the Achievement popped when interacting with "Evil Dead Man" in the Pavlov complex, which aligned with my progression tracker so I don't believe this was a fortunate glitch. This left another 2 corpses remaining in the Pavlov Center that were "Extra", so there should be some wiggle room if you happened to miss an earlier corpse.

There are plenty of videos available so I won't duplicate that here, but what I have done is added the list of discovered Talking Bodies that can be discovered according to the location aligned with story progress, primarily to avoid back and forth in the Open World area. Hope this helps.

Facility 3826 - Vavilov Complex
Chatty Corpse Mandatory - has Lock Code
Dead technician After cable car falls and Burva tunnels ahead of you
Scared Dead Man Inside of an upper floor in the Birchtree area
Trapped Man Heading to Algae Lab, after dealing wiht mutants, objective maker has you going left through a gated door, instead go right and near the dead end under some debris
Chatty Corpse Upper platform opposite the Algae Polymer Insert console
Chatty Corpse On the way to the Pesticide and old Workshop, where the cooridor splits, under a yellow and blue sign
Dead Scientist Room after the windows showcasing the "mother" then the Chicken, and finally the mutants that are accidentally alerted
Dead Florist After meeting the person sumping pesticide on the large plant, decend to the ground floor, this corpse is in the corner under some greenery blockign the path
Dead Romantic When guiding the pesticide container to the plant, you will use telekenesis to reach an area in the center, climb onto the damaged platform and over to the otherside of the room, the corpse is on the elevated railing near a chest in a dark corner.

Forester Area
Dead Loser End of the road, past the elevated train station
Dead Conductor Under the Train Station, behind a small shack near the stairs
Dead Soldier Across the street from the Train Station - has a train ticket
Dead Technician In the upper corner of the building up the street from the Train Station, use the polymer trail to swim into the open window - has a Train Pass

Open World
Dead Game Developer Back corner, opposite the stairs leading down, from the crashed train car when first entering the area
Fallen Corpse South West of the crashed Train car, near the area boundary past the lone building

Exhibition Center - VDNH
Dead Technician On the floor of the second magent puzzle room, back corner near the broken stairs
Dead Engineer On the floor of the third magenet puzzle room, back corner.
Dead Tour Guide First office on the left, opposite the pattern locked Tokamak Complex display room
*Talking Body Room with the Plant item for Tereshkova (FemBot)
Dead Tour Guide Opposite the entrance to the main VDNH facility on the uppr floor.
Suicidal Corpse Main VDNH facility, 2nd floor stair well near some black roots/tentacles.
Dead Tour Guide Left of the "moon" lock on the second floor, near the room's back doorway
Defiant Dead Man Through the doors opened by the previous corspe, at the top of the stairs to the left
Defiant Ichthyologist Pavilov Floor, near the area's chirper on the walkway overlooking the animal organ projection
Taciturn Corpse Past the Plyusch fight, in a lab with several circular operating stations and numerous Lab Tech Vova, its laying in one of operation station bays.
Cranky Talking Corpse Aquarium area, ground floor
Silly Corpse Near the satellite relay puzzle, requried for Claire's head
Drowned Man On the second floor of the Aquarium, climb the crashed Beluga craft to reach.

Open World
Glitched Dead Near the exit from VDNH Complex, the body is on the road but cannot be interacted with (doesn't count)
Dead Weird Scientist Next to the building on the upper area connected to the dock along the river near Testing Ground 9
Strange Corpse Small Crater in the grass near the main staircase (NW) leading up to the Kollective Complex in the Testing Ground 9 region.
Dead Accountant Bathroom of the Sechenov Reseach Centre lobby, where you go to confront Petrov and Filatova, near Testing Gorund 8
Dead Farmer Near the Testing Groung 10 entrance, among some hay bales, near the area's Volan (Hawk's relay station)
Stinky Corpse In the Testing Ground 10 lower region, far south east side past the 2 buildings away from the road, under a pile of manure
Dead Milkwoman Along the lower farm area's road between Testing Ground 10 and 2, just past a tall white wind turbine
Dead Bureaucrat Near Testing Ground 2, below the white platform with the spiral staircase leading to the Hawk Relay.

Lastochkin Theater
*Talking Body Right, after leaving the entrance elevator
*Talking Body Destroyed room on the third floor after powering up the elevator, there is a yellow ledge observed from Chirper location for "What a Performance" you can reach by jump dashing through the partially collapsed wall to a yellow marked floor; above the Mother/Sprout filled room.
Glitched Dead 2nd floor room, under the location from the previous corpse, body is highlighted but cannot interact with. (doesn't count)
Concession Stand Attendant Ballerina Performance Act 1: Space Odyssey area - Behind the concession on the lower level
Talking CorpseBallerina Performance Act 1: Space Odyssey area - 2nd floor, lying next to the pattern locked door leading tells you to find the electrician for the door code
Talking Corpse Electrician who holds the door's pattern code, story related and will be marked with a waypoint unless you enter the code already
Dead Prima *1 There are two bodies in the back dressing room where the all the mutants attack during the performance. I am assuming each corpse is individual and therefore both will contribute towards your progress, may need to confirm.
Dead Prima *2 see above
Lastochkin's Corpse After the Boss Battle, behind the stage in a corner

Infirmary
Talking Corpse 2nd floor of the Infirmary, head up the stairs from the main entrance and it is under debris in front of a blocked doorway
Paranoiac's Corpse Enter the building at street level, this will be in one of the back rooms on the floor near a bed.
Talking Corpse Another body on a gurney on one of the back rooms when you first enter the Infirmary building

Pavlov Complex
Evil Dead Man Pass through the Disk Locked door, in a dark corner behind a gurney with a body on display
Dead Florist 3rd Floor of the Morgue, next to the locked door exit
Dead Doctor Distribution Centre in the middle of the room filled with robots, at the feet of large gold statue.