Been a while since I've had those. 3 chunks will do fine. If you like it smoky, go for 4 chunks. With pork butt you'll be fine either way.
I'm about to put 2 10lb butts in my smoker overnight (which I would suggest as they can take 10-15 hours depending on the butt, then at least 2 hours of rest in a cooler). Anyway, I'm using 7.5 ounces of hickory as I likes it smoky!