I have a Zen Cart which is about 5 years old and all of my products (I have hundreds of them) have been listed without any tally of how many are left in stock. Is there some way to add that on to an individual product?

For instance, if I have a limited quantity of one product and I want to show my customers how many I have left, and I want that quantity to update automatically when someone buys one, how can I do that? Or can I do that?

I am running Zen Cart version v 1.3.9h

Thank you for any advice you can give me.