There were a fair number, but none of the hijackers wanted to use the craft as a suicide bomb before 9/11, usually they were looking for a ransom or to go to a special destination.
The answer is: one
In 1994 a disgruntled employee for Fedex took over a Fedex plane, before other crew members regained control.
(Ironically, he was planning on flying the plane into their headquarters in Memphis.)
Before that, you have to go back to 1978 to find a hijacking that originated in the U.S.
So saying “TSA = no hijackings” doesn’t hold sway.