Always register as much students as you have in your class. We will send you as much student accounts as you have asked us. However, only the accounts which are used count towards the competition. It doesn't matter if you register more than you actually need.
Even if you don't have enough devices for everyone, that is OK. Students just have to take turns. Students need to play with their own login details, otherwise the points in the competition will not be saved correctly.
All in all, you should register as many students as you have in your class and always use the students' individual login details when playing. :)