OMORI

The best way to know how many bunnies you defeated, is to meet Leafie in Vast Forest's Train Station. He'll give you a quest that rewards you items for defeating a specific number of bunnies, such as 5 bunnies gives you a Cotton Ball (DEF +1, SPD +3), 25 bunnies gives you Bunny Ears (DEF +3, SPD +12) and 50 bunnies gives you a Rabbit Foot (SPD +15, LCK +10). Interacting with him, he knows how many rabbits you defeated by just looking into your eyes (a bit weird but that's alright). Even after doing the side quest, he can still be interacted with and given an exact number of rabbits you defeated.

The best spawn is outside the Train Station, because it's close to Leafie and his weird power. There will be three rabbits, one near the Train Station, one in the top right after the first spawn, and one near a watermelon at the top (this one is a Forest Bunny? that is upside down and is a bit stronger than an average Forest Bunny). Re-entering the Train Station, and then exiting will respawn the bunnies.