Happy new year to you too!
As the other coaches mentioned, there are multiple ways to tackle this market sizing exercise.
Something to clarify before starting to think how to approach this, would be to clarify whether the number of planes that are currently operational should take into account commercial planes AND cargo planes.
Assuming that it does not and it focuses only on commercial planes:
One quick way to approach this exercise is by starting with the number of airlines globally. The first step here would be to assume an average number of airlines per country. Then, the second assumption would be around the average number of aircrafts that each airline operates. Multiplying these two assumptions would give you the total number of planes globally.
Another way to approach it would be by starting with the number of passengers. First, you would have to assume active passengers around the world. Then, you would have to assume the average number of passengers that a plane can transport per flight which will give you the number of planes you need to accomodate the active passengers, assuming they only travel once.
Hope the above is helpful!
Thank you in advance!