Catering for 70 ppl --need opinions

If you are preparing the sandwiches and if there are sides, then you will have leftovers at 6 butts. If this is buffet style, then 8. Four ppl per pound and at roughly 50% loss after cooking with an average of 4 pounds net per butt (16 servings per butt).
16 people x 8 butts = 128 servings.
16 people x 6 butts = 96 servings.

Your mileage may vary.

70 people x 5oz per person = 350oz or 21.8lbs of cooked pork x 2 (50% yield) for raw weight = 43.6lbs divided by 8 (average butt weight) is 5 1/2 butts. You will get more than a 50% yield but every once in a while it will be 50%. I always use 50% yield and it leaves a buffer staff to eat or what ever.

70 ppl. Pulled Pork

I would plan on 5-8 oz per person depending on who you're feeding, whether you're serving it and whether it's sandwiches or a plate of BBQ. I figure 50% shrinkage to be safe. So for 70 people at 5 oz/person, you'll need six 8 lb butts raw. For 8 oz/person, you'll need nine 8 lb butts. Both are rounded up to an even number of butts.