If we look at the 2008 statistics for the 30 busiest airports, we already arrive at a value of 1.4 billion passengers in one year.

Divide this by 365 and we're still looking at nearly 4 million passengers a day.

Granted, most of these won't be in the air "at the same time" since most flights are just 1-3 hours long, but the number is still pretty high for sure.