A 2-inch thick slab is about 1.5 yards per cubic foot, so it will take about 150-200 bags per cubic yard.
A 4-inch thick slab is about 2 yards per cubic foot, so it will take about 50-75 bags per cubic yard.
3 inches thick: 125-150 bags per cubic yard.
5 inches thick: 63-81 bags per cubic yard. 20% higher than the thickness in inches because the bagged concrete has a density greater than 1 (note: 10 thick would be 83-100 bags).