Running 100 amp wire for future use - how many wires do i run?
Run a 1.5" RMC

The 60%-of-width limit on bored holes in non-load-bearing studs from IRC R602.6 limits the maximum size of conduit that can be run within a 2x4 partition or furring-wall to 1.5" trade size (2" trade size conduit has a minimum OD of about 2.2", which exceeds the 2.1" limit that 60% of the 3.5" actual stud width/depth sets).

The best choice here then, balancing availability with usable fill, is a 1.5" RMC.  This gives you just enough space, at 533mm2 available vs. 528.9mm2 consumed, for a feeder consisting of 3 4/0 Al XHHW-2 wires, with the RMC as the ground.  This would allow you to bring 175A maximum, or 100A with voltage drop held below 3%, to the garage, which is adequate for whatever you, or just about anyone else, would want there.

Considering this is merely a provision, though, I would simply run the conduit for now, leaving it unpopulated with wires until the garage is built.