How many pieces per person do i need?

I am holding a birthday party for my husband, we are doing a finger buffet supper and basically i am going to get a bunch of chips, dips, bread, cheese ball and then a bunch of appetizers form like M and M's. They say on the boxes how many pieces come in each box but how many should i budget for each person coming? It's an appetizer supper so i wanna make sure they are well fed