One cup of flour equals four ounces. With 16 ounces in one pound, you'll have four cups in one pound, or 20 cups in a five-pound bag.
However, the only way you'll get that many cups out of a five-pound bag is to measure flour correctly. To correctly measure flour, use a spoon to lightly scoop flour out of its container into a measuring cup. Continue until the cup is overflowing. Then use the backside of a knife to level off the flour even with the top edge of the measuring cup.