Depends on whose platoon you're talking about. Unless you're Marians or C*/WOB, there's 4 troopers to a squad. But depending on the faction, there may be anywhere from 3 to 6 squads in a platoon.
Mercenary/IS Generic: 4 troopers/squad, 3 squads/platoon (12 troopers/platoon)
AFFS/LCAF/CCAF/Periphery Generic: 4 troopers/squad, 4 squads/platoon (16 troppers/platoon)
FWLM: 4 troopers/squad, 5 squads/platoon (20 troopers/platoon)
MHAF: 5 troopers/squad, 5 squads/platoon (25 troopers/platoon)
ComGuard/WOBM: 6 trooper/squad, 6 squads/platoon (36 troopers/platoon) (I'm not 100% on this one, but fairly sure)
If I didn't list a military, I'm not sure how they do it.