You set Max players and the Registration closes date in the event wizard, on the Registration step. When the cap is reached, checkout says the event is sold out. After the close date, sign-ups stop automatically. To change either one later, open Ask AI and say what to change. It shows a preview and asks you to confirm.
Want to show urgency? Turn on Show spots remaining in Settings. Redswing does not keep a waitlist for you. See Sold out and waitlists.