Theoretically, there is not a limit to how many players can be in a Multiplayer lobby. The list itself supports up to 32 visible. In the game code, there isn’t a really defined hard limit. 32 would be a safe bet.
I’m interested in seeing how far it can really go if you actually have more than that. We have never truly stress tested the server before. The most I have ever seen in a single lobby is 9 people.
There is no particular anticheat system, but I assume you mean chat system.
Insert will toggle the chat window opened and closed. LeftCtrl+Enter or just “/” will focus the chat window for typing.
All players must /ready or press the Ready button on the screen before the game can start. A room host or member with /op permissions can also force the game to start by clicking the Force Start button or by typing /force.
A list of some more useful commands is available in the /help menu.