You can only have 4 players per team (so 8 players in the game), and those 4 players have to be in the same wing. The wing name suitable for players can be defined, IIRC, in Missions Specs. This doesn't prevent you have have as many AI ships as you want in completely separate wings.
It is possible with some little hacking to make a coop (so with 12 players) behave like a TvT. But don't worry about it unless you really need more than 4 players per team and/or unbalanced teams.