First off, welcome!
It does not matter how many figures are on the stand when representing infantry, wether they're a full 30-man platoon or just a single guy after heavy casualties. In standard play, a single platoon always moves and fires as a single unit. The only exceptions are sub-platoons, which are what happen when you have a platoon of more than 30 men. That platoon is split into sub-platoons, and each of those is treated as a single platoon as far as rules are concerned. In advanced the Tactical Operations rules, there are rules for splitting a platoon into individual squads, but that's an optional rule that is beyind Total War play.