Good question. TSA screens right at 2 million pax per day, and most are domestic, not international. Probably more weekdays, less weekends.
Google brings up an
opposite of FT blog (
please don't tell them about a MR, flying to go nowhere 
) which mentions 300,000 over the US in 2005, which seems about right for a daytime hour if 2 million total fly. I'll guess US is half or more of world total, making 400,000 to 500,000 max in the air on a given day's peak hour, worldwide.