Classroom capacities

For each classroom you can specify the maximum number of students it can accommodate:

Then for each class you can specify the number of it's students.
See also:
Number of students in class and groups
How can I display capacities and students counts on the cards?

Note:
You do not need to specify this information. Specify it ONLY for rooms where you are expecting the problems. Usually it is enough to specify the available classrooms for each lesson.
See:
How to assign lessons to classrooms?