SignUpGenius is great for any kind of reservations where there is a limited amount of "things" or "time slots" that can be specified, and as people sign up, those items or slots reduce in number so things don't get overbooked - leading to eventually if the thing becomes full you can't select it anymore. Is it possible to do the same thing in Survey123? And if so, how? Obviously, the form needs to be smart enough to know what has already been submitted. Not afraid of some light coding if needed (i.e. js functions, arcade, python, webhooks etc).
You might need to go beyond the Survey123 web editor to achieve this. Links below might be able to help you out.
https://doc.arcgis.com/en/survey123/desktop/create-surveys/xlsformformulas.htm
https://doc.arcgis.com/en/survey123/desktop/create-surveys/pulldatajavascript.htm