Managing Class Capacities is done when setting up individual events. When editing a class style registration event, you can configure the individual class dates and times as shown below.
For convenience, you can change all of the class capacities for all of the time slots at once by entering a value in the "Class Capacity" field on the top right hand corner of the form and clickin "Set All Classes" This will result in the class capacity being set to a new value or all dates and time slots that the class is offered.
You may also modify class capacities individually by time slot.
After you "Save" these values to commit them to the database, the class registration tallies would reflect any new values entered here.
If a class were full for example, you might choose to expand capacity by moving the capacity from 24 to 36 - perhaps because you've assigned another coach to offer three classes instead of two. As soon as you do this, assuming registration was active, website visitors would be able to register for the class.