No clue. The free map is supposed to be for 2-4 players. Duels and junk. Judging by the size of the map I was in against some bots, I think maybe 8 could fit comfortably. I would imagine as it's the epic guys making a modern pc game that they'd want people to test it with as many as possible before it's unplayable and/or crashes the host system.
PSA: You need an account with epic or whatever before you start downloading and then playing. It's very unobtrusive, though. Create, get the installer/launcher, download, play.
PSA2: Because of the demise of gamespy, ut3 now needs a small patch to connect to what is now the master ut server list.
http://steamcommunity.com/games/UT3Black/announcements/detail/1659889441679492576