So this seems fairly simple from what I’m seeing and I’ll try my best to help you understand it!
So basically if each square is equivalent to one square foot, and 1 paint can covers 6 square feet, then you basically count out how many squares there are and divide by six. So 20 square feet (all of the dark squares added) divided by 6 (how many square feet the paint covers). So 6 goes into 20, 3 times, with 2 left over. So you’ll need about 2 cans and somewhere around 80% of the third one. Or something along those lines if you want to get exact. But really it’s 3 cans (there is just a little left over).