Can you limit the amount of sign-ups per event?

I checked your form https://www.jotformeu.com/form/50603567183354 and I understand what you were trying have.

You can achieve it using the Input Table field + Calculation widget + payment integration(i.e. Authorize.net payment).

You can test my demo form here https://form.jotform.com/82727124919968

I hope this help. Let us know if you need further assistance.