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It would all depend on how much you like. When I rub my butts, I cover them with just enough rub, so you can't see the meat any longer then I throw just a bit more. I keep my rub in a sealed baggie,so hard to guess at # of cups.
I would say I use about a cup or a little more on a butt. But, I try to stick as much as that thing will hold and shove it in as many cracks as I can. So maybe a cup and a half per butt?
To me, it depends on what type of rub you are using.
If it's mostly sugar, then I would pile it on, becasue most of it will melt and run off.
If it's a more savory rub...mostly paprika, then I would tend to use less because it stays around better.
A cup should do it if it's a paprika rub. Maybe 1 1/2 - 2 cups if it's more sugary..