A 24" deep hole that is 6" in diameter has a volume of (pi)r2h which is
(3.1416)(32)(24) = 678.6 cubic inches. Since 1 cubic yard is 363 cubic inches, That makes each hole 0.01454 cubic yards. The inverse of this (1/0.01454) is about 68.8 holes per cubic yard of concrete.
Stephen La Rocque.>
Dennis, I have a question. Are you going to put posts in these holes? If so then the posts will displace some of the concrete and you should be able to set more than 68 posts with a yard of concrete.