It depends on the thickness and square footage of the slab.
Calculate Square footage of the Slab For 2x2 slab length x width (2x2) = 4 square feet.
let us assume that thickness of the slab is 4 inches Square footage of 2x2 slab = 4 sqft.
40 lb bag yield = 0.30 cubic feet 60lb bag yield = 0.45 cubic feet 80 lb bag yield = 0.60 cubic feet let us choose 60 lb bag having yield of 0.45 cubic feet
No. of bags = vol. of slab/ yield of 1 bag vol. of 2x2 slab= 1.33 cubic feet yield of 60lb bag = 0.45 no. of bags = 1.33/0.45 = 2.95 thus you will need 3 bags for 2x2 feet concrete, 4 inch thick.