Each Waiting Room (WR) can have up to 25 helpers and up to 500 guests. It is a protective logical limit (there is almost no chance people reach that limit in real life).
1) Each Waiting Room is limited to including up to 500 guests.
2) You can add a guest user with the "Add a guest" action or you can remove an existing guest user with the "Remove from WR" action.
3) If you try to add a guest when your room is full, a pop up will appear that says: "You are limited to 500 guests per a waiting room. You can remove some of the existing ones to add other people.".
4) The guest counter is displayed as "<n>/500" on the "Add a Guest" screen.