I can still remember learning this 30 years ago, 1994. Back then, we could j/s on UAL as a 135 operator even without a formal agreement, but we were bottom priority. UAL was only allowed to take as many jumpseaters, as there were jumpseats on the aircraft. No more and couldn’t toss them in the back. I went over to America West, and was told when trying to list at the gate that the jumpseats were taken. I was about to say thank you and walk away to find something else, but the gate agent said there was 65 or so open seats in the back. When I boarded and after checking in up front, went to the back and there must have been 20 other pilots from other carriers jumpseating and seated back there.