It's all in context, and "number two" should not be confused with "number twos" (used plurally, unless one is constipated, maybe). I see no reason to avoid "number two" here.

Choosing Your NUMBER TWO
When President-elect Obama chose Rahm Emanuel as his chief of staff, he picked someone whose aggressive, driven style is a counterpoint to his own calm demeanor. What’s the most important thing you look for when hiring a second-in-command?
http://www.bnet.com/blog/intercom/choosing-your-number-two-b...

I doubt Obama chose a sh**head as his chief-of-staff, even if political opponents might think he did.

Never Take Your NUMBER TWO'S Like This! - Video
www.metacafe.com/watch/.../never_take_your_number_twos_like... May 2007 – Watch this disgusting video of a stupid guy running and going for a toilet break at the same time. Its not a pretty site
[prob. not a pretty sight either]

www.imdb.com/title/tt1030253/quotes
Pam Beesly: So you would be the Regional Manager and the Assistant Regional Manager, Andy is your NUMBER TWO, I would be the secret Assistant Regional ...

This is a meaning I didn't know:
number twos
term used by white people to describe blacks when there are a lot of black around. used mostly by law enforcement officers.
hey arthur did you pick up any of those drunk number twos walking out of the club last night?
http://www.urbandictionary.com/define.php?term=number twos

Arthur? There's actually a law-enforcement officer called Arthur?




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Note added at 1 day5 hrs (2011-11-04 15:55:51 GMT) Post-grading
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Now that I think of it, at his bowling club my father used to introduce me as "Number Two son" (Number One son also being a member of the club) ... I wonder what he meant!