There have been 43 US Presidents. (Barack Obama is number 44, but Grover Cleveland is counted twice, because he served two non-consecutive terms, so really there have only been 43.)

14 (counting the current President, and assuming he survives to the end of his second term) have served all of two (or more) terms.

George Washington, Thomas Jefferson, James Madison, James Monroe, Andrew Jackson, Ulyses S. Grant, Grover Cleveland (non-consecutively), Woodrow Wilson, Franklin D. Roosevelt (served 3 complete terms and died early in his 4th), Dwight Eisenhower, Ronald Reagan, Bill Clinton, George W. Bush, Barack Obama.

Three presidents were elected to a second term, but did not complete it:

Abraham Lincoln (assassinated), William McKinley (assassinated), Richard Nixon (resigned).

Four presidents were elected to and served a second term, but had not been elected to nor served all of a first term:

Theodore Roosevelt (completed McKinley's term and then elected to one of his own), Calvin Coolidge (completed Warren Harding's term and then elected to one of his own), Harry Truman (completed FDR's 4th term then elected to one of his own), Lyndon Johnson (completed Kennedy's term then elected to one of his own).

The term limit was always 4 years, as specified in the Constitution.

There was originally no limit on the number of terms, but tradition (established by George Washington) set it at two terms. FDR broke this tradition. In 1951, the 22nd amendment to the US Constitution set a limit of two elected terms (plus the possibility of up to two years serving as President having taken over from someone else who was elected) as the maximum number of terms.