How to check a game's current players?

Hello. I want to create a Main Menu Game. Then I have 2 Other Games with Maps. They have their Max Players set to for e.g. 30 and I want the Lobby Game to check if this amount is reached and then hide the Button to join.

Back to my question now: How can I save the Amount of Players of a Specific Place in a Variable.

Sorry for my Bad English. :/