I'd like to know how many flights occur on average every day worldwide. This includes all commercial passenger and cargo aircraft, all non-commercial aircraft, private aircraft and military aircraft. A flight consists of a planned take-off and landing.
As mentioned above there are a significant number of aircraft that have no way of being recorded. However, we can make an educated guess. According to the FAA, they support; "Every day, the FAA's Air Traffic Organization (ATO) provides service to more than 42,000 flights" - https://www.faa.gov/air_traffic/by_the_numbers/. These numbers would be aircraft with flight plans, controlled airport landing & takeoffs, flight forwarding, vectoring, etc.
Probably (a guess) at least as many general aviation aircraft do training, VFR flights, helicopters (i.e. ranch cattle herding), balloons, scientific, skiing, hunting, sightseeing, gliders, ultralights and maintenance flights without filing a flight plan or interacting with ATC.
So that puts the US at about 80,000 flights per day. The US is approximately 5% of the world population and if we use that as a basis for calculating flights, then there are about 1.6 to perhaps 2 billion flights per day, including all forms of aircraft.
This is interesting - we might at first conclude 1/4 of the earths population is flying on any given day. This would be a wrong assumption because many passengers embark on several flights to reach a destination each day. For an example, a sightseeing cross country trip might include 3 or 4 stops that include re-fueling and eating, or restroom breaks. But, this is still a staggering number of people flying each day. Conservatively, perhaps 1 out of every 50 to 100 people are traveling by air each day!!!