I am a "big gal" as Bent says and I have learned that I can only eat 2 ribs when there are sides.  If there is also brisket, I don't see most woman taking more than 1, but absolutely no more than two. 

With brisket, I can't see most men ever taking more than 3 and probably just 2.

15 racks at 13 ribs a rack is 195 ribs which seems more than enough with all the other stuff to eat.

Around here at a restaurant a typical rib order with 2 sides is 1/2 rack (6 bones).
A 2 meat plate is 1/4 rack (3 ribs), 4 brisket slices.
60 people at 4 servings pre rack = 15 racks.
I like to have a buffer just incase uncounted guests show up.
I would cook 18-20 racks. Better to give out take home plates than apologize to the late arriving guest. 

I was always taught in catering in a situation that will be self serve, put the sides & bread 1st, make folks fill up their plate before they get to the main course, at least give them less room to be a pig when they do.  You may have no control over that, or you may at least be able to put it in the ear of the person that does...if they don't listen, just make sure you tell them you cooked enough for the amount of people that are showing up!

Defiantly will cut into singles thanks guys, we will not be "serving" it will be self serv. BBLEW you think people will actually take two bones or you think there eyes will be somewhat bigger than there belly's when they see BBQ ribs ?

I think I will make sure brisket is on the table first

Lots of great advice.   I'm not going to offer any advice, just an experience of the last time I tried to feed 60+ people.  My only job was the meat.   Three 8 lb bone in pork sholders, three 8 lb chuck roasts.    Cooked to perfection and pulled.   Problem is that this was the main dish, and for the most part people didn't get to it.   It was put out last.  With drinks and sides, I'd estimate I was 70% too much meat.  So you really need to access your audience and sides and beverages.   It throws a wrench into x lb per person calculations. 

DK

15 racks sounds like a safe approach.

What will you be cooking that many ribs on?

Was hoping that wood master would see this and offer up one of his mini hogs for rent 

Actually I am doing half of them and my buddy offered his smoker i could use also so it will be my memphis pro, my charbroil smoker, buddy's smoker, and son-in-law as a backup

Thats probably my biggest concern right now is how to get 15-20 racks of ribs done all at the same time

Thanks all for the input definatly appreciated it.. Good idea Bentley

LTS

It would not be my preferred method, but I did do 103 Racks on 2 Traeger 075's in 2005 or 2006 at a cook for Marines at 28 Palms Base.  We ended up foiling all 103 and they came out pretty darn good, I was proud to serve them, that was the bottom line.   I believe this was the unit with 51 on it...15 Racks, you would have ouddles of room...but these were St Louis cut Spares...