In total, there are 14 garages to open in South Park. However, only 4 of them actually require keys to open. (The garages are house garages which mean they are all attached to houses)
Cartman's Garage Key - The key is in the drawer as soon as you enter Catman's house.
Butter's Garage Key - The key is in Butter's house upstairs in a drawer next to the parents bedroom.
Kyle's Garage Key - The key is in Kyle's house upstairs in his parents' bedroom in a drawer on the right.
Jimmy's Garage Key - The key is found during the quest in Jimmy's House/Giggling Donkey upstairs between the two doors in a drawer.
This is missable as Jimmy's house becomes locked later in the story.