How to put an amount limit

I have a multiple choice section where parents can sign up their child for lessons.  I need to put a cap on how many can sign up at each different time.

Is this related to the field 'Please choose class date/time according to your child's level'? 

If so instead of the single choice field I would suggest you to use the Inventory or Ticket Purchases widgets if you also need to perform a calculation inside of the form. 

How to Add a Widget to your Form?

How to Setup a Payment Order Form with Limited Stocks

Let us know if you need further assistance.