We typically remove the form from the registration page and replace it with a message explaining that registration is full. There may be other/better ways to do this in an automated way.
Hi Macarena! If you follow me on Community, we can exchange info (we aren't allowed to link to email address or websites from Community posts).
It's been on and off the roadmap for years. I think the PMs have put this in the too hard basket seeing as there's a lot of third-party solutions out there. But please everyone keep voting on the idea. I raise this at every Summit in the roadmap Q&A session just to annoy the new breed of product managers. (Hi guys!)
I had a client need to do this recently and we were able to come up with a not perfect, but workable solution for automating it.
We created two tokens: one for the current count of registrations, and one for the maximum count of registrations. Every time the form was submitted, +1 would be added to the count of registrations token value, and every time the page loaded, it would run a piece of script that looked at whether the count of registrations was the same as or greater than the maximum. If so, it would redirect to a waitlist page.
For many reasons, not a perfect solution, but if you've got some developer resource and the holes aren't too risky for you, it could work. For our clients use case - fairly low volume of traffic, mostly directed to internal staff - it was worth testing (I will note that the program hasn't yet run to completion so we don't have feedback yet on how well it worked and whether they'll use that solution again).