Add a space that classes can be scheduled into, and set how many can run in it at once.
Needs the Manage schedule permission.
Event limit is how many classes may run in that room at the same time. A studio that only ever holds one class takes a 1; a hall you split down the middle takes a 2. Leave it empty and Orlo never stops you scheduling into the room.
The limit is checked when a class is created, not when one is edited. Moving an existing class into a room that is already full is not refused, so it is worth looking at the week before you start rearranging it.
That is Group size, and it belongs to the class, not to the room. You set it on the class itself when you add one to the calendar, and on each entry in the Weekly schedule, which passes it on to every class generated from it. Two classes in the same room can take different numbers of people.
So a room whose event limit is 1 still takes as many bookings as the class in it allows. If a class is filling up sooner than you expect, the number to change is on the class, not here.