How can I limit the number of peple that can choose a selection?

You can do this with widgets, there are some of them that will allow you to set a number available of selections or items. 

Here're the links: 

https://widgets.jotform.com/widget/inventory 

https://widgets.jotform.com/widget/appointment_slots

https://widgets.jotform.com/widget/gift_registry

https://widgets.jotform.com/widget/quantity_gift_registry 

This guide will help you to add a widget to your form: https://www.jotform.com/help/252-How-to-Add-a-Widget-to-your-Form 

Hope this helps.