How many female managers existed in the 1950s?

Only a very small percentage persons in management positions in the 1950s were women.

It depends also what you call a "manager."  If you count someone running their own small restaurant or shop a business, then a larger number of women could have been counted as "managers."  A December 1955, article in FORTUNE magazine, titled, “Who Is an Executive?” estimated that there were roughly 250,000 "real" executives in the US, and that the number of those who were female was certainly less than 5,000.