If you mean 80# premix bags, I suspect that the manufacturer is a little closer in their estimation. The pre-mix bags yeilld little more than a 5 gal. pail when mixed. I assume you need to plaster the lath on the wall as well, which will use a good amount of mortar.
Without knowing all the details, or how well you handle a trowel, I'd guess around 20 - 80#'s if you're using pre-mix.