The NEC doesn't have any limit on the number of receptacles in a residential garage application, but as mentioned some areas may have local codes that supersede it so you have to check that. Also, there's common sense in not putting a zillion of them on a single circuit.
Also as mentioned you can use 15 amp receptacles on 20 amp circuits, as long as there are more than two of them, and a standard duplex receptacle counts as two. Devices that require a 20 amp receptacle are pretty uncommon and most people don't have a need for them.