Dead Rising 3

This achievement is missable as any of the survivors can die. There are five survivors located in each of the four areas of Los Perdidos for a total of 20. There is no in-game tracking for how many you have saved or which ones.

You can complete this achievement at any time, but I would advise saving at least Jack, Madeline, and Bob and any others that you may encounter on your route to Cora before you even get in the van at the begining of this DLC. I had Bob die, without his quest popping up, just by driving past him to deliver the van to Cora. Another time I had the van blow up while trying to save a survivor, despite having the Indestructible Vehicle attribute (this happens with all story-related vehicles).

Here is a breakdown of all the survivors located in each district:

South Almuda
Jack - Intersection across from power plant
Mila - South of the Wrench O Rama
Maia - In the middle of the freeway rubble in western South Almuda
Vicki - South of the tunnel to Ingleton
Ryan - Just west of where the Chemical Van is located

Central City
Madeline - To the left after crossing the bridge from South Almuda
Bob - Fountain near the tennis courts in southeast Central City
Sam - In front of the Wrench O Rama
Jordan - To the left of the museum
Alec - Entrance to the highway from Central City

Sunset Hills
Tracey - East end of the highway near quarantine/biker area
Eva - To the right immediately upon entering Sunset Hills from the south
Greta - In front of te Wrench O Rama
Jasmine - Across from BiBi's Box in western Sunset Hills
Alex - Outside the under construction safehouse in Sunset Hills

Ingleton
Scott - Outside motel safehouse in northern Ingleton
Mike - Northwest corner of Ingleton
Kristen - West of the Wrench O Rama
Kaliyan - To the right of the south exit, near U Break Wi Fix
Steven - North exit of tunnel to South Almuda