How much meat should I buy?

We're having a potluck at my office next week. I want to make my grandmother's meat (I swear they're the most delicious things ever created). I make my meat medium-sized. The largest number of people who could be there would be about 30 (I'm expecting less than that to show up). How many pound of meat should I buy??? I don't want to look cheap but I don't want to come home with enough meat to feed an army.