Well, I cannot tell you the exact number, but here is a site that shows a live radar of all the aircraft in the US flying. From the airlines to the general aviation aircraft.
http://flightaware.com/live/
The sky
In the sky, you can see clouds, the sun, the moon, stars, and sometimes airplanes.
A formation
Sky magazine is published by delta airlines as a service to their customers. I have found no connection between sky online and sky magazine.
There where three plains that where ordered out of the sky on 9/11.
The engines create smoke .
Condensation trails or 'con trails'.
Some people have drawn things in the sky with airplanes, some gas or smoke but its only temporary.
-- stars -- meteors -- airplanes.
The sky appears blue due to Rayleigh scattering, where shorter blue wavelengths of light are scattered more than other colors by molecules in the Earth's atmosphere. This scattering effect is present regardless of whether airplanes are flying or not. Airplane contrails can contribute to haze under certain atmospheric conditions, but they are not the primary reason for the sky's blue color.
exhaust
there are 1,ooo,oo1 airplanes