After I did like 2 Scouting Missions, it seems they dropped out of rotation until I did a combat mission?
That shouldn’t happen. Still, I’ll do some test-runs on my end to see if I can observe this oddity myself.
Also, how many credits do I need for Sector 14?
Unlocking sectors/planets is not a matter of the player’s credit balance. It’s a matter of clearing story missions. I believe the in-game map gives precedence to standard missions in regards to it’s display, though, it’s also true that players don’t “start” with a story mission either. It has to be found among the sectors/planets that are initially available.
This decision of having the player find the game’s story mission arc on their own might be clinging to it’s Wing Commander - Privateer inspiration too closely for it’s own good.