A five-pound bag of all-purpose flour can contain anywhere from 15 to 20 cups of flour. This estimate will vary slightly depending on the type of flour, the brand name and whether or not the flour is sifted when retrieved from the bag.
Thicker and heavier flour types, such as whole wheat or rice flour, are going to contain slightly fewer cups of flour per five-pound bag as compared to lighter flours, like white all-purpose flour. Also, sifting the flour can cause the flour to contain more air volume, increasing the number of cups you can retrieve from a five-pound bag. Since flour can easily be packed down to push out air volume, packing down each cup of flour can greatly decrease the number of cups you retrieve from a five-pound bag.