You meant merman?
Anyway I think it should be some blacksmith, because someone should repairs Zorro's swords.
At the beginning of One Piece there was flashback of Zorro as a bounty hunter's. Zorro was told, that he overuses his swords and one day he may completely destroy them. That's why they need someone to look after Zorro's swords.
Moreover the blacksmith could make Zorro some special sword.
Anyway, I didn't found anywhere in the manga Luffy saying about 10 crew members.
It was indeed in the anime, but if I remember correctly Luffy said something like: 'I don't need to many crew member. I think 10 ppl should be enough for the beginning.'
But I don't have dvd's with One Piece at the moment so I can't check if I'm right.