How many U.S. presidents have been the sons of previous presidents?

There have been two pairs of father/son presidents. John Quincy Adams, the sixth president, was the offspring of John Adams, the second president (who, on learning his son had won the election, remarked ‘No man who ever held the office of President would congratulate a friend on obtaining it’). Far more recently, we had George H.W. Bush and George W. Bush.

Source: History