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:
- Emirates: 4,300 pilots, 258 aircraft: 16.7 pilots/aircraft
- Delta: 14,600 pilots, 881 aircraft: 16.6 pilots/aircraft
- United: 12,491 pilots, 763 aircraft: 16.4 pilots/aircraft
- Southwest: 96,14 pilots, 748 aircraft: 12.9 pilots/aircraft
- SkyWest (US regional airline): 4,621 pilots, 453 aircraft: 10.2 pilots/aircraft
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.