How much rub do I need

5 lbs of rub minimum. Maybe more if you put a lot on.

Sauce - depends. I can get nearly 4 butts in one full steam pan. That's about 16 lbs of pork and I can add at least a half gallon to that. And you'll want extra for people to add if they want. And I always use the same sauce for beans.

Unless you plan on foiling all of those butts, I would not do much more then salt and pepper on them. Rubbing those butts will leave you with a hard almost inedible bark unless you have a super moist cooker. I for one have a moist cooker si I can cook without foil all the way, just be real careful it can turn to a lot of waste. If I am cooking high heat and cannot foil I will add rub after pulled for the flavor.

I do cooks like that on a regular basis at 275 on my FE's. Takes about 12 hours and stays super moist. I don't foil.