How many hives are needed for pollination per acre for different crops?
The main resource for information about pollination of specific crops is called Insect Pollination of Cultivated Crop Plants by S. E. McGregor.
There is a section on each of your crops with contain research information on hives/acre. The county extension service in your state may have information specific to crops grown in your area. To locate your county's extension office, go to your state in this list: http://www.csrees.usda.gov/Extension/USA-text.html