How many stories high would 20 million 1-dollar bills stacked atop each other?

A "story" isn't a fixed unit of length, on average a story is considered to be 10 ft.

The thickness of a dollar bill is 0.1 mm, 20 million dollar bills would create a stack of 2000 meters (approx. 6561 ft) so it would be 656 "stories" high.

But this is of course an approximation.