At the creation of the class, you have to enter the capacity. It is possible that sometimes you may decide to increase or to reduce the class capacity. To do this, you can follow the instructions below.

Go on 'Classes' > 'View All', select the season that contains that particular class, and then click on the class you want to amend.

Once on the class page, on the tab 'Basic Information' tab, press 'Edit':

Then, in the 'Other Information' block, you can change the capacity of the class. Don't forget to 'Save' your changes.

This means that if you have online class enrolment activated you can control if students are enrolled on the class or are added to the waiting list if capacity is reached.
Find more info about online class enrolments here.