It'll take more than you'd think due to fat and moisture loss. You'll probably lose 40% of your raw weight during cooking.
I did a search and found this pork shoulder
thread that has really good info, including a link to a catering calculator.
The amount of meat you'll need will vary a lot depending on what else you're serving, and the makeup of the crowd. 40 grown men will eat a lot more than a mixed crowd of men, women, and children. 40 male high school seniors will eat as much as 80 grown men, I'd guess...
I generally figure on about a 1.5 pounds of uncooked meat per person, but that's because I'm obsessed about not running out of food and I love to give away left overs. Most people say to figure on about a pound of uncooked weight per person, which generally allows for a safe margin after cooking even for big appetites. Anyone who says something really silly like 1/2 pound uncooked weight per person is probably a vegetarian.