It would almost impossible to calculate all the aircraft in the air at any given time I heard from somewhere that in their heyday that there was a PA aircraft departing or arriving at an airport around the world every 5 seconds. and thats just one airline.
"There you go ladies and gentleman we're through Mach 1 the speed of sound no bumps no bangs... CONCORDE"
Not to mention all the military flights, general aviation flights, etc...
But if you are just talking commercial, Star Alliance alone generally has about 1000-1500 planes up in the air at any given time, and that's just one alliance. It's probably upwards of 10,000 but Star Alliance is pretty big though. But if I had to guess, I'd say roughly 10,000.