There have been many single term US presidents. Far smaller, though, is the number of presidents who, after being elected, chose to not run at the end of their first term. In fact, there are only three:
James K. Polk – promised to only serve a single term if elected
James Buchanan – realized he had messed up the Union and left it for Lincoln to deal with
Rutherford B. Hayes – promised to only serve a single term if elected
If we include those presidents who served less than a single full term, there is only a single president who did not run for re-election: Andrew Johnson, who cut his losses after being impeached.