To calculate the number of sheets needed to cover a wall, you need to know the wall's square footage.
The square footage of a wall divided by the square footage of the sheathing (one 4ft x 8ft sheet = 32 square feet) equals the number of sheets needed.
If you have a small opening on your wall, then ignore that. Run the sheathing through and cut out the opening afterwards.
For larger openings in the wall that don't need sheathing, subtract the square footage of these openings from the total square footage. However, you may waste some of the sheathing material, and you may need more than the calculated amount.
wall length = 12 feet wall height = 8 feet size of the plywood = 4ft x 8ft
8 x 12' = 96
This wall has a square footage of 96 square feet
4 x 8 = 32
A 4 ft x 8 ft sheet of plywood has 32 square feet of area
96 ÷ 32 = 3
For this wall you will need 3 sheets of 4ft x 8ft plywood.