Use your best judgement and eyeball it. IMO, on a pork butt, if I was deciding between 3 or 4 pieces(just an example - not a recommendation) I would pick the extra piece. Apple wood will be milder than hickory all things equal. Make SURE you write it down somewhere for next time. Have fun. The worst PB I've smoked has been really good.
There's no rule against mixing woods. I like a little hickory or pecan with apple.