This is a broad topic because it varies so much depending upon aircraft type and utilization. Asking about fleet utilization per route could be very complicated, especially for shorter routes, but I'm trying to get a feel for how many aircraft an airline would need to open routes of varying distance and frequencies from multiple airports.
Assume average fleet utilization with minimum "downtime", scheduled inspections, spares, etc.
Are there any general guidelines to follow? I'm sure it would be far easier to determine if one had knowledge from starting an airline and gradually growing larger, but for larger airlines, it seems that for somebody unfamiliar with these practices, it's a daunting question. I have read on these forums that about 3 aircraft are necessary for a reliable ultra-long haul route such as EWR-SIN or ATL-JNB. Those are obviously extreme examples, and easier to determine. How would these look for smaller segment lengths, such as 5,000nm, 4,000nm, 3000nm, 2000nm, etc...down to regional networks where aircraft are generally never more than a couple hundred miles from a hub or maintenance base?