I would like to share my approach to the question number of commercial passenger aircraft worldwide.

1. #Passengers per year / 2. #Passengers with one aircraft per year.

1.1. #Passengers in developed countries + 1.2. #Passengers in undeveloped countries
to 1.1. => 1.1.1. #Passengers per year for leisure flights + #Passengers per year for business trips
1.1.1 
Assumption: every 5th lives in a developed country => 1.6b., 10% belong to the upper income class, 50% to the middle and 40% to the lower.
upper class: 160 m; of the upper 10%, each flies 2x per year (assumption based on personal observations)
middle layer: 800m; from the middle layer everyone flies 1x per year - 1x every 2 years = on average 0.5 per year
lower layer: 640m: from the lower layer everyone flies every 5 years => 0.2x per year 
= 320m + 400m + 128m = 888m passengers per year for leisure flights

Business trips: 50% of leisure flights = 444m

= approx. 1.3 b passengers per year

1.1.2. 
Assumption 4/5 live in undeveloped countries and only 5% of them can afford to fly = 320 mio, people fly 1x every 2 years (assumption based on personal experience / reading) 
= 160m passengers per year

1.1. = approx. 1, 5b passengers per year

1.2.#passengers on one aircraft per year = #passengers that fit on average on one aircraft x #flights per year per aircraft wing

Assumption an average passenger aircraft has capacity for 300 passengers

Operating days: 36 (5 days for maintenance and breakdowns)
14h / day an aircraft flies from Frankfurt to New York) 
Assumption: 80% are domestic flights 20% (based on observations at the airport)
=> 4.5 x 0.8 + 1.5 x 0.2 = approx. 4 / day 
4 x 360 = 1,440 / year

300 x 1,440 = approx. 430,000 passages per aircraft per year

1.5 b / 430,000 = approx 3,500 aircraft