S.C.
It depends on what all else they have to eat. You can probably make do with 12-13 pounds IF there are a lot of other things to eat and you put out smaller buns. BUT leftover sloppy joes freeze GREAT, and one of my favorite things to eat in the world is pizza made with leftover sloppy joes, onions, green peppers, mushrooms and co-jack, so I'd round up and get two of the 10 pound tubes of ground beef, Plus one of those 10 pound tubes of ground beef fits PERFECTLY in my crock pot so I'd use it (and borrow my step-mom's crock pot that's just like it) and make and serve the sloppy joes in them.