There are 45 80-pound bags and about 60 60-pound bags of concrete in a cubic yard. Each 80-pound bag of concrete covers about 0.6 cubic feet. There are 27 cubic feet in a cubic yard: 27/.6 = 45.
Though the number of 80 pound bags is exact at 45, overestimating the number of bags needed is a common construction practice to account for the minor variations found in building materials. In addition, most building applications do not use perfectly cubed yardage of concrete, as most slabs are poured just four to six inches thick, and footers for deck support tend to be cylindrical.