I work at a glass plant that runs 24/7 and we need 4 shifts to run 24/7.
I would also think the same thing for skymarshalls since planes take off 24/7.
I bet we will need 100,000 skymarshalls to cover 24/7"
Nope, just enough non-uniformed marshalls that don't disclose their identities. If you aren't sure which ones have the cops on them, you'd be hard pressed to want to hijack one. Otherwise, just allow the pilots/crew to carry.
The era of all hijackings has ended, and no government measures will deserve credit for it. The change in mentality among passengers will deter almost all future hijackers, and prevent any more plane-as-missle efforts.