In Ngalinn, there are 5 factions that offer the misions, though the factions often switch them off for an hour or so, so sometimes it might appear that only three of them have them. Get yourself a notebook and write down the name/code for each faction, then write down how many pirates you have to kill next to each faction that you got the mission from. That way, you can keep the totals balanced. The goal is to get the same number of pirates from each faction. You can plan the length of your sortie what your totals are. You get about 30 to 40 mission targets an hour - less if you get a poor spawn and more if it's a good one. if the spawn is too slow, logoff back to the menu, then log back on.
to get the highest pay-outs, you'll need to spend some time refreshing the mission board. You get new missions every 10 minutes on the exact 10 minutes of the clock, like 9:00. 9:10' 9:20 etc.; however, there might be some sort of algorithm that knows what you have and it might to adjust the board away from what you want, so you have to make a judgement of which ones to take and when. For that, you need a bit of experience because the board is different for different players depending on rep, rank and possibly other things.
The best missions pay 41mil for 30 pirates or 20 mil for 15, but I think you only get those if you have Elite combat ranl and are allied with a faction. You will see the pattern of what you can get. Non-wing ones always pay more per pirate, so take those first.