"Das Mädchen kennt die Zahl nicht."
Wouldn't you use Wissen in this case since it's a number? Hence, having knowledge.
"Das Mädchen wisst die Zahl nicht."
You mean "Das Mädchen weiß die Zahl nicht." Both "kennen" and "wissen" are fine here.
I think that you're wrong and that dj1121 is right. The difference between "kennen" and "wissen" is that "kennen" means "to know an object or person" and "wissen" is "to know a fact". This is a 'wissen' situation....
I second wataya. Depends on context. http://german.about.com/library/anfang/blanfang16b.htm
*Not a native speaker though, so my opinion could easily be disregarded.
So this would mean the kind of number that would be the answer to a math problem? Whereas Nummer would mean phone number or address?
I am not quite sure, but I think that the German sentence would have been "Das Mädchen kennt die Anzahl nicht."
Yes, or "Das Mädchen weiß nicht wie viele." Note that in this sentence you can't use 'kennen'.