The usual serving size for meat for a person is 1/4 lb if it is served to them - you control the portion size. If people are allowed to serve themselves, you can up that to almost 1/2 lb per person. Most people will make bigger sandwiches. I usually figure 1 sandwich per female and 2 per male. It does give me extra food but I would rather have too much than too little. As for Mostaccoli I figure a cup per person.
The other thing that you could do is to go to Gordons Food Service (GFS) and they will tell you exactly how much to purchase - a free service if you purchase the food there. There is one in Joliet, Orland Park, Mattesson and probably further north also. They are a restaurant supply service that is open to the public without any membership fees.
Also, when purchasing your beef, realize that a 5lb container usually contains about 2 lbs of meat and 3 lbs of juice!
S.