Current rules require that a pallet position be left open as needed to accommodate passenger baggage if there are 20 or more passengers. Therefore 20 or more passengers would mean (at least) one less pallet position available for mission cargo. Therefore, for planning purposes, 19 seats are normally listed in advance and adjusted as needed nearer flight time depending on required mission cargo. AMC is considering removing the requirement to allow units the flexibility to decide when a baggage pallet may or may not be needed based on available space on the aircraft and the number of bags.
Why do I often see 19 seats listed on the flight schedules?