Would probably take a day or so to work through this (and I am not going to right now!) but probably something along these lines.
Start with a World Airliner Census (there are several available).
Take 800 Boeing 747 (worldwide fleet), times (390 (average number of seats times) times 75% (average load factor)), times 13/24 (because they average 13 hours a day in the air.
Gives number of people up in a 747 at any one time. Result = 126,750.
Repeat for all other types. Actually not that many. Then throw something in for General Aviation and Military (these latter two spend over 95% of their time on the ground).
Challenge for somebody with nothing to do !