How many crew members that Straw Hat should be?
What the initial quote was "about 10 people" so there is no hard and fast rule regarding the final size of Straw Hat Crew. 
I'd like the theory of 4 more people joining. 4 (Zoro, Usopp, Sanji, Nami joined in East Blue), + 4 (Chopper, Franky, Robin and Brook) in the first half of grand line

Among the next 4 I am thinking Jinbei is a given. That makes 10 but I'd like to think some other allies such as Caesar, Vegapunk etc. may also join. There is this really vast theory about the following members joining which draws parallels between the previous joinees and the future recruits


   Jinbei, Smoker, Unknown Warrior probably from Elbaf and Tashigi. I liked it especially since it filled the "old enemy Smoker" and another female recruit as well as a fishman. I also agree with the theory that Luffy's garp is Coby. 


About Oda's quote, Unconfirmed rumours are floating around for ages. Next recruit is an old enemy, next recruit would be a female (Cue why people wanted Monet with Logia powers or Baby5) etc. etc.

http://www.marineford.com/Thread-One-Piece-Eiichiro-Oda-Interviews-Statements
This is a compendium of Oda's interviews that I find is well researched.