The limit on how many players can join a game is likely to be the upload bandwidth on the host. The engine itself imposes no limit, but there is definite practical limit that will be encountered.
The problem is the host has to send a message with data for N players, to each of the N players. For example, if the host needs to send 16 bytes of data for each player, then each message will have a size around N * 16, and then that message will have to be sent N times. This creates an N-squared bandwidth requirement. For example:
10 players = 16 x 10 x 10 = 1600 bytes per update
20 players = 16 x 20 x 20 = 6400 bytes per update
30 players = 16 x 30 x 30 = 14400 bytes per update
100 players = 16 x 100 x 100 = 160000 bytes per update
Even though the player count increases linearly, the bandwidth requirement increases proportional to the square.
Even with a significantly more powerful server or less data needed per-player, it won’t get you many extra players. Crap