Time's Up! rule to guess a number
I would assume a normal charades-style ruleset applies to the mimed round of Time's Up.  Looking at the "Rules of the acted charade" as given on Wikipedia I see the following: 


  Most commonly, the actor is allowed to make any gestures other than
  blatantly spelling out the word. In more stringent sets of rules,
  indicating anything about the form of the phrase is prohibited, even
  the number of words, so that only the meaning may be acted out.


I don't see how there can realistically be a prohibition on holding up seven fingers to "mime" the word seven.  Any more than it should be illegal to mime "head" by pointing to one's head!  It just so happens that numbers are unusually easy concepts to mime...