Prime Rib questions

I did a small standing rib roast for about 4 or 5 of us last Christmas and it turned out fantastic.  Thus I have been drafted to do it again this year for Christmas.  I do have some questions though because instead of cooking for 4 or 5 I will be cooking for approximately 17 this year.

How big of a roast do I need?  I remember reading that one rib will serve approximately 2 people but I thought that most rib roast come with 7 ribs.  So will I need one whole roast and a smaller partial?  That might work out because one of the guest is pregnant and has to have her meat at least medium.  Don't worry that is not her typical preferance. 

I dry aged last year's roast in the normal fridge prior to cooking and I am planning on using an old dorm fridge to age it again this year I was hoping to go for about 45 days.

I really have some questions about the logistics.  What time should I put the roast on to eat around 1200hrs or 1300hrs.  We will be going to a Christmas eve service at midnight and I have a BBQ guru so maintaing temp is not really an issue.  I think I should pull it out early evening on Christmas eve to let it come up to room temp prior to roasting.  How long of a rest does such a large cut of meat need 30min-60min?

Does anyone have a good au jus recipe?  Last year I used Alton Brown's from  his good eats episode and I wasn't too pleased.  I think I screwed up because I used a Pinot Noir because that was what I had on had and the sauce tasted too much like the grapes in the  wine.