How can I track a field based on the number of record in another module?

How can I track a field based on the number of record in another module?

Hello!

I am currently on the product page, where it says "Qty in Stock". I'd like it to show a number that reflects a certain number of records in another module, say "Inventory".

Records in Inventory are pages relating to the individual products. 

How can this be done?

Thank you!
James