By 1940, the SAT was taken by over 10,000 college applicants, and the surge in college enrollment occasioned by the G.I. Bill and other factors would lead to rapid growth in the number of test-takers and institutions that accepted the test through the 1950s. By 1961, the SAT was administered to over 800,000 students each year.
https://www.manhattanreview.com/sat-history/