Answer:
Only one. Gerald Ford was the only president who was not elected President and also was not elected Vice President. Nelson Rockefeller (Ford's Vice President) was also not elected to the Vice Presidency.
Under the terms of the 25th Amendment, Ford was appointed Vice President when Spiro T. Agnew resigned as Richard Nixon's Vice president in 1973. When Nixon subsequently resigned in 1974, Ford became President. Rockefeller was appointed to be his Vice President later in 1974.