OK, let’s look at this again.
On a typical day, for every 1 flight by a scheduled air carrier there’s about 5-10 flights by general aviation.
So, if we have a day with, say, 15,000 schedule air carrier flights we may also have 75,000 GA flights, for about 90,000 flights in a single day.
If the weather is exceptionally fine, there might 100,000 GA flights to go with those 15,000 scheduled lines, for about 115,000 flights in a day.
Now add the military flights - training, cargo, mission, etc.
Now… were those 15, 000 scheduled flights just for North America (in which case we might hit 1,000,000 or more in a day) or world-wide?
The problem with statements like “millions of flights each day” is that weather has such a huge impact on flying. Today, for instance, in my area nothing other than scheduled airlines, cargo carriers and the military are flying, and even then are experiencing delays because the weather around Chicago is absolutely crappy. On a truly fine spring day, however, a turf strip in the middle of nowhere that usually sees 5 planes a week might see 50 in a day, paritcularly if someone is cooking a pancake breakfast next to the runway. There probably are days with over a million flights, but I don’t believe it happens every day.