Is there a window I can add to show how many units are left after customers buy one?

Please checkout Inventory Widget here https://widgets.jotform.com/widget/inventory

I think this is what you require.

Here is a demo form that shows the widget in action https://form.jotform.com/61471146729964

You can see the 'available' quantity in the description of the items. That number diminishes every time an item will be used.

Let us know how we can be of further assistance.