Luffy also said (after getting Zoro) that the next nakama would be the musician, but Brook came much later on. Not all he says must be taken seriously, I think.
It would make a lot of sense if they were 10, though, because Blackbeard's crew has 10 people too and I do see them fighting each other at some point on the story. It's not entirely necessary, though, since the SH fought the CP9 in Enies Lobby with one person less, remember? Usopp didn't dight anyone, Luffy fought Blueno AND Lucci.... who knows what Oda might come up with.
As regards who could be the tenth nakama, it's not going to be Jinbei. Sorry to disappoint you, but he doesn't have a dream to fulfill by travelling with the SH nor does he have a position to fill in the ship. And don't come up with "he wants peace between fishman and humans" because that's no dream, and if so, he would fulfill it from FI, following Otohime's dreams to make people fill the forms and travel to the surface. And he would restore the fishman forces and make peace between fishman first. Jinbei's not leaving FI. It's the same case as Wiper from Skypiea and Paulie from Water 7.
The tenth nakama will probably be introduced in the next arc (given that he/she exists), no one presented to the moment fits the category. Except for Vivi, who I would LOVE to see back in the crew. But I must face reality, she will stay in Arabasta.
*sobs*
