How can I limit the number of items that can be entered into a list?

You may be better off listing all sessions in one list with spots available

user registrations using a forms which go into another list, this form has a lookup to the sessions list with a filter to only show sessions with spots > 0. And a workflow on each registration to take a spot available from a session each time a person registers.

This would need to be an enhancement to out of the box forms using javascript, powerapps or another 3rd party forms product