How to control inventory when doing Print on Demand?

Hi,

I am using Printful as a supplier, however I would like to limit inventory . Lets say, I want to put 2 units for one product, once those 2 units are sold. I would like to manually put that 2 more units are available and so on. In order to get comfortable with the procedure.

I have reached both Shopify and Printful support teams, but neither of them knows how to do this.

Thank you,