I would love it if there were a way to pre-determine the number of leads who can fill out a form and be directed to the registration confirmation thank you page! Perhaps after the cap is reached you could determine your secondary follow up page (ie. sorry this event is full, you've been added to the wait list.)
The only workaround now is to use a webhook to write to a custom integer field and add choice for send email (send confirmation if less than cap, send wait list notification if capped) and change program status (live event>registered if less than cap, live event>wait listed if capped).
Even with this solution, you have to send yourself an alert when the field value is equal to the cap # and then manually swap out the follow up page.
.png)