It's expected that Obama will announce tomorrow that Jack Lew will be replacing Tim Geithner as Treasury secretary. Starting with Alexander Hamilton, there have been 43 presidents but 75 Treasury secretaries. Lew will be the 76th.
Below is a chart of the number of Treasury secretaries by presidency. Two term presidents are highlighted. Surprisingly, there have only been 13 presidents to serve two full terms. On average each of those presidents has had 2.5 men serve as the head of Treasury (there has never been a woman serve), so there's a decent chance there will be at least one more before Obama's done.
FDR, who served longer than any other president, including for the bulk of the depression, only had two men helm Treasury. Meanwhile two of the four presidents who had four Treasury secretaries weren't even full one term presidents. Tyler and Arthur both took over from men who died in office.