A quick Google shows about 9000 Boeing commercial aircraft in service and 5100 from Airbus. So 14,100 airplanes, and I am going to guess that 75% of them are in the air at any given time and that each holds an average of 150 people. So:

1,586,250 people in the air.

Just a guess, but I imagine it is not too far off... certainly the figure is at least a million.