What is the best way to approach a 'How many items do you have selected' type counter?

What I'm meaning is, I have a Table View and Populated Cells, I want each cell to have a button/checkbox that switches (On or Off State). When it's in an 'On' State, it adds +1 to a counter, when it's clicked to it's 'Off' state it takes back that count (so subtracting).

Imagine a 'How many items do you have selected' type of counter.

I'm unsure/can't find info on how to do this without programming each switch individually, I imagine theres an obvious way and I'm just not privy to the keywords.

Thankyou