A lot of large team games are tournament games, or created for a group of friends. However, I've seen 14+ team games in the open lobby.
FFA games pretty much either way. Quite often I've forund them by accident, or been invited by a friend, or join a tournament.
If your wondering what a Tournament is check out wiki if you haven't already, It covers alot of questions new players have.
Wiki -- http://wiki.warlight.net/index.php/Main_Page
Tournaments -- http://wiki.warlight.net/index.php/Tournaments