Specifically, the server's upload bandwidth.
Too many inexperienced people using <say, cable> think they have 6 Megabits (Comcast, my ISP US advertised average) per second.
Though 6MB would be nice for a server, the upload is usually less than one megabit. 384k... about 5-6 people tops.
In a perfect world though, CPU and bandwidth not counting...I'd say 15-18 people if they mostly all knew each other.
Less if they don't. Too many people on power trips, even if they don't have admin.