There is no minimum requirement for the number of rest stops located along interstate highways. The wording in the document that established policy for safety rest areas on the interstate highway system called for a sufficient amount to be built to reasonably accommodate travelers.
The Federal Aid Highway Act of 1956 officially created the interstate highway system, officially known as the Dwight D. Eisenhower System of Interstate and Defense Highways. Shortly after, in 1958, the American Association of State Highway Officials created a policy paper on safety rest areas for the interstate highway system.
In this paper, the association deliberately chose not to create a minimum number requirement for rest stops, or a minimum average distance between stops, due to several controlling factors such as traffic volume, topography, and climatic limitations. As a general guideline, it was decided that safety rest areas were to be established, so that in combination with other stopping opportunities, an available short stop is feasible for travelers every half hour of driving time.