Thanks for the responses guys. The variance is quite interesting. The number I'm after is to be used for a business plan.
Aircraft types are CJ2 and Lear 60. (Not the actual one but these two fit the generic profiles quite perfectly in terms of cabin size and capabilities).
I too would think that the average would 2-3. I can't quite possibly think of parties larger than 3 (on a regular basis)-although I definately would LOVE to see that.