It is as Weirdo says.
Also, a few notes. For one, this doesn't apply to the limitations of a campaign setting. This is- in a perfect world- exactly the way you would like to play. Secondly, there's no rule that, in a campaign setting, each player must play only one character. Granted, especially if you want to get really in-depth nitty-gritty with it, it does help keep everyone in-character and reduce the odds and ease of metagaming, but if you're running a vehicular-scale, say company-level campaign with RPG character creation and advancement, plus RPG-exclusive stats and rules to enhance such a campaign, there's nothing to stop you from saying that each player may create and control 2-4 MechWarriors. In fact, in these sorts of campaigns, I tend to find that dual-character play helps keep players engaged even after losing a character to an unfortunate death (especially if they die at the beginning of a session). So yeah, no, you shouldn't need 12 players for a 'Mech company or 84 players for an infantry company.