How many pilots does an airline hire for each new plane they purchase? 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