Registration from

I'm creating a registration form that has 20 slots available, but I need to create 2 different categories that add up to this 20 slots is there an app that will allow me to do this. 

You can use the following widgets:

Appointment Slots

Inventory

Gift Registry

Quantity Gift Registry

Inside of this guide, you can find how to add a widget to a form:

How to Add a Widget to your Form?

You can also limit your form to 20 submissions. 

How to Limit Submissions in a Form

Hope this meet what you are looking for.