For limited registrations (a class, a trip, limited tickets), open Settings, turn on Limit number of entries, and set a maximum.
- The public form shows "N spots left" and stops accepting once it's full, showing "Registration is full."
- The cap is enforced safely even if lots of people submit at the same instant — it will never go over your limit (no overselling).
- On payment forms, a spot is counted when the payment goes through; on free forms, when the form is submitted.