Limiting the number of times a value from a list can be selected
Hi, I have created a database for class registration. I am offering three sessions each on six subjects, and I want to limit the number of participants for each session to 20. I will assign people to sessions.
Is there a way to limit the number of times a value from a value list can be entered into a field? I want "class a, session 1" to appear in the "session" field a maximum of 20 times. Ideally if I were to enter a 21st occurrence I'd get a message, or it just would not let me make the selection (which would be fine).
Help is appreciated!