You need 6 for team stuff IIRC and 2 for CFA. However there is a Challenge for being top on a team of 6 so that requires at least 9(?) if you set it up right.
Also you can add hundreds of hours onto these numbers if a Challenge glitches on you which happens a fair bit. Hence rand al Thor 19 hasn't finished it. He was pretty much done with the stuff you need to be playing for when a survival Challenge glitches on him. The only way to fix is to delete your save a start all over again. I had it happen early on only losing 30hours or so.
Also if you don't frequently back up your save chances are you will never get it done and will be constantly restarting as the game a sick fetish with freezing during autosave which corrupts your save file.
Also everything is stored on the save file including the MP stuff. NOTHING is stored server side like other games. So lose the save and lose everything. I had 2 back ups going while I did this.
But the biggest advice I can give is if you are a completionist, avoid this game like the plague. The SP wasn't bad and can net you a good chunk of GS in a few hours. But I would seriously advise against ever touching the MP.