I have a hunch that something bad is going to happen to jinbei before he officially joins the crew.
He'll be incapicitated at the very least and won't be able to travel w/ everyone once he's done with whatever he's doing.I say this b/c the sh crew would be way too big once they hit double digits. Sure there's plenty of room for jinbei on the ship, but 9 crew members seems perfect.
4 joined in east blue and 4 more joined in the grand line.Not that I wouldn't mind the SHs to have the worlds strongest fishman in their crew, but there's no room for another character.
What about Luffy claiming he wants at least 10 members? and the fact that he still asks every interesting person (or animal) that they come across to join the crew. I think its pretty well established that there will be 1 more crew member.
PS and the fact that the Green Databook says that there will be a new crewmember. case closed.