Law Regarding Animal Numbers

From the USDA Animal Welfare Act regulations and standards, Section 2.31- 
“(e) A proposal to conduct an activity involving animals, or to make a significant change in an ongoing activity involving animals, must include:

Identification of the species and the approximate number of animals to be used;A rationale for involving animals, and for the appropriateness of the species and numbers of animals to be used;”

From the U.S. Government Principles For The Utilization And Care Of Vertebrate Animals Used In Testing, Research, And Training: 
“III. The animals selected for a procedure should be of an appropriate species and quality and the minimum number required to obtain valid results. Methods such as mathematical models, computer simulations, and in vitro biological systems should be considered.”