Generally, the airline that has the contract for a team, has contracts for many other teams too. The Bills fly United, and United has a lot of NFL charters. It's easier to manage if the teams have their own dedicated light numbers every week. Direction, length of trip, etc., don't change the flight number. Sometimes the contract will call for a larger plane for long haul destinations, but even then they use the same number every week. Once you get the number from flightaware, it should not change throughout the season.