Create a counter when user starts from 0

I am creating an inventory management app and all of my users start out with 0 inventory until they can find which parts they have (which I am providing for them). In the app, they are shown certain suppliers or parts where they can choose specific items they can add that will go to their inventory tab. I have a part description page that a user goes to when they click on a specific part and they can add or subtract to update or delete their inventory. I’m trying to figure out how to implement this in my Database and show up in the UI to populate the sum of the input they have plus their starting inventory which for everyone starting out will be 0.

Thanks so much! This is the last piece that is holding me back from completing what I want to get done. Any guidance would help!

So if you look below: The user can go to this screen after they pick an item they have. It will show the current stock level text with an ellipse and the current stock level which will be 0. I have the input for quantity working with the arrows but after the Update button isn’t working. I want each user to keep track of each item stock level in the database but also updating it if they use an item or add an item. Thank you!! Here is my user database also.