Including William Howard Taft, who was President in 1912, and Barack Obama, who was President when this answer was posted, the total is 18.
There have been a total of 18 presidents since 1912.
20
13
12
8
The Federated States of Micronesia has had 7 presidents since 1979.
As of May, 2012 there have been 11 U. S. Presidents since Harry Truman.
5
44
A lot
There has been only two democratic presidents elected since Ronald Reagan, they are Bill Clinton and Barack Obama.
23
Including the one in office in 1920 and up to and including Barack Obama, there have been 17 U.S. Presidents.