If the 20000 is sq/ft of floor then you need to measure the wall linear feet. There can be a lot of walls in 20000 sq/ft of floor space. If the 20000 is the sq/ft of base then the linear feet is 5000.
This how I got that figure – Total sq-ft / width in feet;
* sq-ft x .25 (3 inches=.25 in feet)
* 20000 x .25=5000 ln-ft
Or, there are 144 sq-inches per sq-ft, 12×12. 3×48=144. 3 inch tall base at 48 inches long is equal to 1 sq-ft. 48 inches=4 feet. Divide 20000 by 4=5000
Most base will be delivered in 16ft lengths. This will be 313 pieces of base.
It has been a long time since I done this so double check my ideas. If you are looking at a set prints always measure the actual wall linear feet for accurate figures.