The answer will depend on how large and area you want the topping for. Without that information, no sensible answer is possible.
81
55
How many square yards in a 20x18 feet 3 inches thick
one (1) cubic yard of concrete four inches thick will cover 80 sq. feet. 85 yards x 81 = 6885 sq. feet.
A cubic yard is 46,656 cubic inches. Spread to a thicknes of 5 inches, a cubic yard of concrete covers 9332.1 square inches, which is 64.8 square feet.