Classes can have up to 50 students in it. If you try to add more than 50, you will not be able to add any students to the class.
If you have less than 50 students in your class but are unable to add any more, you have ran out of subscription places.
Subscription places are assigned to classes. Any students in that class will have access to premium features. Subscribed classes have a yellow star next to it.
You can decide which classes are subscribed.
Subscription places are shared between subscribed classes.
Example: you have 30 subscription places. 2 of your classes are subscribed. The 30 subscription places will be shared between the two classes. You could have 10 students in one of the classes and 20 in the other. However, when all the subscription places are used up you will not be able to add students to either of these classes.