There is no way to know that. Even if we could accurately calculate the number of flights airborne at any given moment, not all seats on those airliners would be filled. In the USA alone, over 51,000 flights are tracked every day! According to the International Air Transport Association. it is estimated that 6 billion people fly annually worldwide. Try dividing that out by 24 hours in a day and see what you get for a number 'airborne at any hour'. That will be the closest you will get. Flight tracking software helps with finding the number of airborne flights over the United States but abroad coverage is very spotty and in some places it is non-existent. Without that tool, even finding the number of planes airborne would be impossible much less number of passengers on board them- which only the airline knows. Here is a map showing the number of flights airborne in the US at any time- http://flightaware.com/live/