What is a typical ratio of airline pilots to aircraft?
A Google search shows United Airlines has 709 aircraft and 12505 pilots for a ratio of 17.6 : 1.

If an aircraft was operated 100% of the time, a month's activity (720 hrs) would need 7 pilots pulling 100 hr/month duties, so it's an interesting related question why they need twice this amount. Perhaps there's significant variation in the hours worked by pilots - not all want to work 100 hrs/month, or can be efficiently scheduled to do so because of their home locations etc.