The normal thickness of concrete in driveways is 4 to 5 inches. Using a thickness of 5 inches, here is the yardage:

18 ft x 21 ft x 5/12 ft = 157.5 cubic feet

Since 1 cubic yard = 3'x3'x3' = 27 cubic feet

Hence, the 157.5 cubic feet is equal to 157.5/27 cubic yards = 5.8 cubic yards.

Allowing for waste, you should order 6 yards of concrete.

Hope this answers your question.