I guess this would depend if the airplane is narrow body or wide body because wide body airplanes make longer trips. Another way to ask this question is what is the ratio of airplanes and pilots in an airline company for wide body and narrow body passenger jets respectively.
The answer depends greatly on the airline's operations, as the constraints are very different between long-haul and short-haul trips, whether relief crews are required, layover times, need and distribution of reserve pilots, and other factors: some pilots employed by an airline will be performing management duties, on leave, training, etc.
Assuming the numbers on airlinepilotcentral.com are accurate, we can look at some averages:
Well, that will depend on variable information sources.
For example: how many pilots are currently working in this company? How many aircraft? Do you have cargo or only passenger aircraft? Is it a single aisle or double aisle aircraft? What is the average flying time per pilot in that company? What is the average flying time for this airline per month?
So I think it is way easier if we took it on bigger scale (worldwide).
Accordingly to statista.com, currently active pilots around the globe are 290,000 pilots, and accordingly to telegraph.co.uk, there are from (23,600 - 39,000) aircraft around the globe. With easy math, you can have an answer of 7.4 pilots per aircraft. But again to answer your question we are going to need to assume some of the parameters are fixed or given! And yeah, I have been told that an A320 aircraft needs 10 pilots on average.
Again from market research into the flight training industry, own work. From beginning 2020 just before the covid slump.
- Single aisle passenger aeroplanes require between 3 and 5 crews per plane, so 3-5 FO's and 3-5 captains. Depending on average flight duration, airline growth, local legislation, worldwide job market. So every new additional single aisle aircraft requires an additional 3-5 type rated crews. If the new aeroplane replaces an older type, the existing crews can be type rated for the new type and no additional pilots need to be hired.
- Dual aisle, longer range planes require at least double that, up to 12 crews/plane. For the longer flight duration of these aircraft there need to be multiple crews on board, and upon reaching the destination they cannot immediately fly the return flight, they need to rest.