If the main part of the meal is meatballs, like meatball sandwiches, I'd use at least 1/2 lb per person. If you're doing meatballs in a meal like spaghetti I'd do 3-4 meatballs per person. For regular old meatballs on a bed of oven steamed rice I'd use more like 1/3 to 1/2 lb per person. It also depends on if it's men, mixed adults, families with kids, etc....each of those groups will eat meat in different amounts.
Are you going to serve the food on plates already made up or do family style dining passing the food around or buffet style? This makes a huge difference. Putting how much food you want on their plate then handing it our makes you have total control on how much they get, buffet style is a free for all where they could actually take all the meat and leave non for the others.
I'd say plan on 1/2 lb per person and if there are any left over you can do meatball sandwiches a day or two later or just freeze them for a few days too.
I would just get a 5 lb chub from Walmart and be done with the trying to figure it out...lol. That should be enough.