Correct me if i'm wrong but Luffy said he wanted a 10 man crew, and i'm pretty sure the captain doesn't count himself because he's the leader. SO in fact we have 8 members and he needs two more. But then it wouldn't be too far-fetched to think he could go for a crew of 12-15. Why do people think its not probable? It would just be 4-5 extra people really, nothing big. And i'm pretty sure the Going merry will be revamped soon enough to accommodate their growing numbers. The shipwright has been studying after all.