I have not figured out yet what determines how many missions you see on a bulletin board.
I started to think the lesser populated systems would have less missions. And I think to an extent that is true, because the more populated a system is, the more factions there are in that system, thus more missions to choose from.
I became allied with two factions in Aiabiko, and the amount or rewards of their missions didn't really change.
I'd like to know how the mission distribution works, and how you can get better and higher-paying missions.