Is there a way to designate how many of an item are available on an order form

We have leftover sweatshirts. I would like to create an order form that lists the number available next to the size.

In your current configuration, it's not possible. You will have to go for a workaround such as using the inventory widget to configure the product items.

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

This means that you will have to add the items one after another. You have sub-product items, this also means that you have to add them individually. 

You can clone the demo form for you to see how the inventory widgets are totaled. 

Demo form: https://form.jotform.com/61471146729964.

How to clone: https://www.jotform.com/help/42-How-to-Clone-an-Existing-Form-from-a-URL. 

Let us know if you have questions about the setup.