Several
As many as it can carry.. It depends on the type of airplane. There are small airplanes for only one person and new jumbo-jets that can carry over 750 people in the right configuration. Ultimatly it...
There isn't a way to tell this yet. We are going to be developing a method. Stay tuned!
You take the total population of the world and deduct how many of them are on the surface of the earth at any given time.
http://www.internetworldstats.com/stats.htm