Set limit for each answer

Can we set the limit for multiple answers? For example , max. 20 places for the first choice. The question can be closed after this.

You can not set a limit of answers on a field.

I would suggest you to take a look on these widgets, they could achieve what you need.

Gift_registry

Quantity_gift_registry

This guide will help you to add the widget to your form.

http://www.jotform.com/help/252-How-to-Add-a-Widget-to-your-Form. 

Hope this helps.