Mathematically, six sheets. But in the real world, staggering joints to reduce squeaks and seam problems for a finish material, and maybe some two-by to sister up to exposed joists for better bearing, maybe a few more bucks.
Fun with numbers:
How many posts are needed every eight feet to build an 80 ft fence?
Eleven, if you don't forget the one on the end.
Better yet, if two guys can dig a big hole in two hours,
how many would it take to dig that hole in two seconds?
Before enlightenment, you have to chop wood and carry water.
After enlightenment, you have to chop wood and carry water.