The following information comes from Five Acres and Independence by M. G. Kains. Five Acres and Independence is also available to purchase in print.
Number of Plants Required to Set an Acre of Ground when Planted at Given Distances
To determine the number of plants to set at distances other than shown, multiply the two distances apart and divide 43,560 sq. ft. (to the acre) by the answer. For instance, to set a peach orchard with trees 20′ apart each way multiply 20×20 and get 400 sq. ft. Divide this into 43,560 and get the answer 108.9. (See various chapters on planting.)