gl, I found that the batch of Jan's Rub I made several weeks ago was about enough for 4 eight pound pork shoulders. I applied Jan's, refrigerated the shoulders over night, then put in the smoker. The next go round, I'm planning to apply a second light application of Jan's before the shoulders go into the smoker, so may only get three shoulders per batch.
I think your triple batch for eight butts is going to be about right. And it never hurts to have a bit of rub left to dress the pulled pork for serving, either.