Quite often you need an engineering degree to use some of the available calculators.
To calculate the amount of sheets needed you need to know the square footage of the wall.
Most common wall sheathing to use on a stud wall is either made out of plywood or OSB (oriented strand board). Structurally it makes very little difference in which type you decide to use. On average OSB will be cheaper than plywood. These sheets come in 4' x 8' size and the most common thickness to use on a wall would be 3/8" thick. On occasion, say if you wanted to use a fibre cement board type siding as a final finish layer on your wall, use 1/2" thickness plywood or 7/16" OSB. Depending on your on centre (OC) stud spacing, say if it were to be 2' OC then the additional thickness will make the application of the siding a better looking job. In some cases not quite as wavy.
Measure the length of your wall and the height of your wall. Multiply these together to get the total square footage of your wall.
Example wall length 12' and wall height 8' 8' x 12' = 96 square foot. A sheet measures 4' x 8'. To get the square footage of the sheet multiply 4' x 8' = 32 sq ft Take the square footage of the wall and divide this amount by the square footage of the sheet. 96 / 32 = 3 . It will take a total of 3 sheets to cover your wall.
If you have a very small opening in your wall then ignore that.
If there is a 3' x 7' door opening and 4' x 5 ' window then calculate the square footage of these openings. 3' x 7' = 21 sq ft and 4' x 5' = 20 sq ft. Add these two amounts together 21 + 20 = 41 and subtract this combined amount of 41 from your total wall square footage.
96 sq ft of wall - 41 sq ft of openings = 55 sq ft. 55 sq ft is now what you need to cover. 51 / 32 = 1.59. Now it will take 1.59 sheets to cover your wall. Originally there was no waste involved to cover the wall, but with cutting around the openings you will end up with some useless pieces. But in this case since, most likely you can only buy the sheets in 4' x 8' size 2 sheets should be enough for this wall. With different walls and openings you will have to consider adding a waste factor.