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