Before you host a server, when you're on the main menu, open the console with your tilde (~) key and enter:

g_multiplayer_limit XXX

Just change XXX to a number (like 6) and then host the lobby.

For example if you wanted an 8 player game you would enter:

g_multiplayer_limit 8

Note that officially the game supports 4 players and you may experience instability with larger numbers.