I'm not a big-time pro like some of the others, but I've done about a dozen catering jobs, including one at a church...
I used to work on a half slab of baby backs per person, but that has proven to be too much on average... I would tell you to think about 4 bones (or 1/3 rack) pp of baby backs. Now, spares (e.g. St. Louis trim) have more meat on the bones, and you might do with about 3 bones pp. Sounds light, but I'm telling you: there are more ladies out there who will have 1 or 2 pcs than there are pigs like me who will have a full rack
On the sides, think about 3 oz of cole slaw, 4 oz of pot salad or beans pp. My experience, anyway.
BTW, ribs are relatively costly... pulled pork is always a hit, plus it is cheaper, plus (a big plus): you can prepare it in advance, pull and sauce it, then refrigerate it in chafing pans. Just heat up for an hour in the oven beforehand. Let's see... I get about 1.5 butts (ave. 5 lb butts) per half tray, or 3 butts to fill a full chafer tray. A full tray will make a good 30-40 sandwiches... or more.
There was a great site on figuring amounts for catering... here it is:
http://www.ces.uga.edu/pubcd/b1039-w.html check the chart on BBQ requirements: meat, near the bottom for amounts.