I'm sorry this isn't a finished solution you can just copy and paste. It's more of an example, sharing THAT this can easily be done which may help you figure out how to do it your case. Lots of people at the University of York are using Google Forms to allow people to sign up to events. They use forms rather than Appointment Slots because they want to work with the data to generate registers for the people running the events. But often these events have a capacity, that is, once 20 people have signed up to them, they're full. There isn't much you can do with Google Forms to "live lookup" data and change form items if they're full, so we have developed workarounds to mimic this behaviour. Firstly, having created our Form in the regular way, we create an extra sheet that keeps a track of how many people have have signed up, like this... The count column has a formula in it like this... = COUNTIF ( 'Form responses 1'!G:G , ...
Collaborative Tools Project