Hey there, Rysnt11!
Wow, that is quite an enigma you've brought out. As Gangspear stated, there are, in essence, a maximum of fourteen players in Warcraft. The twelve-player slot limit is merely the limit assigned by Blizzard to human and computer players. The missing two are quite mysterious, and I don't know for sure what they are. A guess I do have, though, is that perhaps Blizzard assigned each and every entity in Warcraft (units, doodads, destructible e.t.c) to a faction. If this is true, it could mean that there exist invisible 'doodad' or 'destructible' factions that may account for the two missing players. Still, this is purely speculative, and is highly unlikely, since why would doodads/destructibles have an owner? So, I'm not entirely sure, but I'm just sharing a theoretical opinion.
I haven't tried your method, but are you sure all your trigger components and parameters are correct? I might try it myself.
Oh, and, in answer to your question about a 7v7 game, that is nearly impossible. Nearly, I say, because getting the two neutral factions to fight - passive and hostile - would require a heck of a lot of triggering, and would be unable to be controlled by players, anyways, and would, in essence, be a waste of time.