Maximum capacity Form



I'm making a form for people to fill in if they want to participate on a event. I made the form and set some dates that they can choose.

The only problem is that on each date there is limited capacity. So what I would like is that when for example they choose 15/7 15 times than the date wouldn't be available anymore for them to choose that date. So I want to set a maximum capacity on a dropdownlist. ( or should a make a date list? )