Which constitutional amendment, ratified in 1951, limits presidents to two elected terms?

Which president is counted as both the 22nd and the 24th President? Which president is counted as both the 45th and the 47th President? Which two presidents served non-consecutive terms? Which president served the longest, being elected four times? Which is the only president to resign from office? Which scandal led to Richard Nixon's resignation? Which president was never elected as either president or vice president? Which president pardoned Richard Nixon?