The answer really depends on the kind of people you're serving. 13 men are going to eat considerably more than a ladies bridge club.
For butts, I figure 40% loss between raw and finished. From a seven pound butt, you'll get maybe four pounds of meat. Allowing four ounces of finished meat per person, that butt would provide approximately 16 portions.
Go ahead and cook a second butt. It won't take any longer than doing just one and you'll be assured plenty of leftovers.
Enjoy!
Ken