building on Sandford's reply. We've built something like this for a client quite recently. It used Flowboost (a JS webservice) to handle the counting of registrants, and then some custom JS code on the landing page containing the form to populate the list of timeslots available based on the counter values.
@SanfordWhiteman wrote flowboost, but won't mention that as it'd violate community guidelines. It's a freaking awesomely helpful/useful tool.
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