As you can probably tell from my username, I run a small business which is a local ConerShop, I am looking for a way to stock count take my total cigarettes stock level in real time, via one or several Rasberry Pi's hooked up.

Looking for any advice that could point me in the right direction on how to start capturing stock volumes from each of the below devices, (sliders). I have about 7x14 sliders in my tobbacco gantry and would like to have a constant count of subtractions(sales), additions (restocking). I have pictures below, each slider contains 7 packs usually.

Apprecite any help anyone could provide.