How to switch from booking form to waiting list?

a client offers trips for a grand total of let’s say 20 people.
booking is made via a registration form.
as soon as the trip is fully booked (= 20 people have filled out the registration form) all further people should be prompted to join a waiting list (via a form) for trips that will be offered in the future.

I wonder how this can be done cleverly/automatically?
any ideas?

PHPJabbers will no doubt have something off-the-shelf for this. Or you could make your own form with Machform.

I suspect you could do something using stacks too, although it might well end up pretty disjointed nd a bit feature lacking.

For stuff like this you are always going to be to look outside the RW world, but for something that can easily be plugged into a RW site.

Great suggestions. Thank you @TemplateRepo
I found this on machforms:

Limit Submission
Automatically disable the form when the number of entries reaches the limit you set.

This should work in companion with a second form for the waiting list on the same page.

You can most definitely go better than that. Machform is really powerful, so I bet there is a way to apply some logic that says if all spaces are booked, just take an inquiry etc.

There are a few more experienced with MF than me on here, so I’d fire up a post. And of courswe get in touch with the devs, who are very helpful people.

That would be even better.
I am really impressed by machforms from what I can get from clicking their website… bookmark deluxe!