I plan for 50% yield after cooking (its probably closer to 60 usually, but I toss a lot of fat and other nasty looking bits). Then plan for 1/4# finished product for small eaters, 1/2# for big eaters, or 1/3rd pound for middle of the road. I'd cook 8-10 butts.
Also be sure to have a plan on how to pull it, keep it at a safe holding temp, potentially cool it, etc. 40 pounds of cooked pulled pork is a lot.