I personally have never booked 15 for any restaurant, actually I think 15 may have been the most we have had down there, but everybody just went off and did their own things.
I have eaten there quite a few times and I have seen large families like yours there, so booking for 15 people should not be a problem. Since LeCellier is really popular call first thing, also if you cannot get a 15 person reservation try breaking it up with like 7/8 or 6/9 for two seperate reservations. This way you can get everybody there, and its just an option in case....Good luck with your ressies tomorrow and hope you don't spend 3 hours on phone like some others have...