Booking list

I'm a bit stuck trying to create an app matching pupils to instructors.

What I want it to do is this:
1. An instructor signs up, inputting their name and the date(s) they will be available
2. Pupils chose which instructor to go with on which day, BUT with a limit of three pupils per instructor per day.
3. Have a list view, showing signed-up instructors and the pupils who have signed up to each so far.

I can't figure out how to make the limit...