In general terms, you put the number recieved (or manufactured if you make them) in a number field and then subtract the total Qty from your portal.
Total Drums would be a calculation field such as Sum ( PortalTable::Qty)
and Drums In Inventory would be NumberReceived - Total Drums (or NumberReceived - sum ( PortalTable::Qty) )
Note that I am using "portaltable" as a placeholder, Use the name selected in "Show Related Records From" in portal set up for this portal in place of that term.
Awesome! Using the "Sum" function was the key. Thank you.