Calculating the number of studs needed per wall 

Similar way to do it is take 20 feet X 12" inches = give you total length of wall in inches (240"). Now if you're spacing studs 16" center, take 240 dived 16" give you 15 + 1 == 16 total for upright studs.

Multiply the lengthy by 0.8 for 16 on center 20 ×0.8 = 16 studs