How many times has the vice president become president because the president died in office?

Fourteen:John Adams (VP to Washington)

Thomas Jefferson (VP to Adams)

Martin Van Buren (VP to Jackson)

John Tyler (VP to W.H. Harrison)*

Millard Filmore (VP to Taylor)*

Andrew Johnson (VP to Lincoln)*

Chester Artur (VP to Garfield)*

Theodore Roosevelt (VP to McKinley)#

Calvin Coolidge (VP to Harding)#

Harry Truman (VP to FDR)#

Lyndon Johnson (VP to Kennedy)#

Richard Nixon (VP to Eisenhower)

Gerald Ford (VP to Ford)*

George H.W. Bush (VP to Reagan)

*- became President when predecessor died (or in the case of Nixon/Ford, resigned), but NOT re-elected [5]

# - became President when predecessor died, then re-elected [4]