how many soils needs to be dug out in order to dig a well of depth 7 m and radius 2m?
how much soil. "many" is used to count individual items, and "much" is used to measure a quantity such as volume.
Recall that the volume of a cylinder of radius r and height h is
v = π r^2 h
Now just plug in your numbers. Volume is in units of m^3.