Many states have policies limiting the number of students per class. View state requirements (EdWeek.org). To comply with your state's policy, you can limit the number of students who can register for a class.
The online registration form reflects these settings and only displays classes that are still available. The total enrollment displays beside the class size limit. This includes the total number of students registered as well as any student registrations that are pending online.
Setting Class Size Limits
You can also change the size limit for a single class by entering it in the class schedule record.