We'd see a plane hijack roughly once per year. Though it wasn't common for the hijacker to try and kill everyone on board.

about once per year but it makes no difference what happened pre 9/11 - it matters what we did to ensure that 9/11 doesn't happen again

Lots in the 70s but by the 90s it was very infrequent in the US. And most hijackings did not result in casualties (or very few casualties). People didn’t hijack with intent to kill the selves and everyone on board. Probably more people died from the shooting down of planes by militaries than by hijackings.