Hi again!
If this is your case:
Newegg.com - NZXT Apollo Black SECC Steel Chassis ATX Mid Tower Computer Case - Computer Cases then yes, 120mm is your magic number
You can have three, one in front, forward of where your hard drives are, one in back, and one on the side
Before you buy them though, check to see what you already have, no sense in wasting money if you don't have too. Also, generally, the front fan should be intake (blowing inside the PC Case) and the rear fan exhaust (blowing out the PC case), but I don't know the protocol when a side fan is thrown in, as I don't have one...
I imagine you'd be best to have the side fan as intake and the front/rear fans as exhaust