If I had to guess, up to say no more than 1 cup per butt. But it really will depend on how much they soak it up (how dry were they).
So reheat them with about that amount, but have a warm pan with some extra in it.
You should adjust the liquid and also add some more rub as you're serving to give it a little kick. I like to grind up my rub a little to sprinkle it on like salt. I taste it and adjust the liquid/rub as needed.