I need to make pulled pork to make sandwiches to feed about 40 people. I need advice on how many pounds of pork butt that has a bone to buy.

Over-estimate. You can always freeze the leftovers. Considering other dishes being served 4 oz is a good portion of finished product. Think of a second portion for 20% of the guests rather than over-sizing the sandwich and watching good food being tossed out.

Is the pulled pork the main event or will there be a lot of other sides? if the pork is the thing, i personally would probably go for about 1/2 to 3/4 pound per person. If you think about it, at Thanksgiving it's typical to plan for approximately a pound to a pound and a half of turkey per person, allowing for leftovers as well. And pulled pork is addictively delicious, so... :)

You need about a 15 pound pork butt. If you use a pok shoulder then about 10 pounds would give you enough shredded meat for 40 sandwiches

I would guess that 1lb shredded pulled pork for a sandwich would feed 3.