How many second ladies were there?

Actually, this is a very interesting question. The use of "Second Lady" to describe the wife of the Vice President is relatively recent; we see it used for the first time to apply to Richard Nixon's wife Pat, when Nixon was the VP for President Eisenhower in the 1950s. But it was not a very commonly used term, and still is not anywhere nearly as common as "First Lady," referring to the president's wife. (And in fact, the term "first lady" did not come into common use till the early 1900s.) So, if you are asking how many wives of vice presidents there were, there have been 47 vice-presidents; but not all of them were married while in office, and further, as noted previously, the term "Second Lady" was rarely used by the media to describe them.