Scott,
If you figure on 6 oz. per person, that works out to 37.50 pounds of finished meat or 75 pounds pre-cooked weight. Some will eat more and some less, but 6 oz. is a number I've seen thrown around by a lot of caterers. I did pulled pork for my brother's wedding with 80 guests and 6 oz. worked out nicely with 3 pounds or so left over.
I've done three butts per rack and I used wooden skewers to keep some air space between them for maximum smoke penetration. Your situation will not allow you to cook it all at once, but you can easily do one round the day before, crash cool the pulled pork after pulling and packaging in ziplock freezer bags in a cooler of ice water. I use a one gallon bag per butt after adding a small amount of No. 5 sauce and a 1/2 table spoon of rub. The next day when you are a couple of hours away from the scheduled dinner time, reheat in a large Nesco roaster with some apple juice. Use some forks or bear claws to separate the meat so that you don't get any cold spots. I've set the heat on the roaster to 250 or so and I've also placed a sheet of aluminum foil over the top of the roaster to retain as much moisture as possible because the roaster lids have holes to release steam.
Pulling that much pork the day of the event isn't a bunch of fun anyway. Good luck with the party. You'll get lots of compliments and it's nice to see that many people enjoying good que.
Tony