There are a total of 14 garages to open.
4 are locked and require finding the key to open
Cartman's, Butters's, Jimmy's, and Kyles.
- Kenny's is also "locked" - the garage door isn't locked but there is a door behind that which is. I am not sure what exactly counts but you need to go into the locked door anyway during the main quest on day one.
Each key is found in their respective houses.
Cartman's key - the drawer right inside the house, to the right of the couch.
Butters's key - inside the locked house in the far left upstairs bedroom (parents). The far right drawer in the room holds the key. To get the house key you must first meet Cartman.
Jimmy's key - during Kenny's rescue in the Giggling Donkey (main quest line Day 1), you can find this in a drawer of the upstairs hall between two doors. Either pick it up during the quest or you can return later to get it.
Kyles key - you must be invited to see the Elf King first to gain access to Kyle's house, this is during the main quest line of Day 2 after getting everything to become Goth. Inside the house and upstairs, the left door is the parents bedroom. Inside the room the far right drawer has the key to the garage.