secrets from a caterers kitchen is a book you should read-one thing it talks about is food&beverage amounts because NO ONE can ever know how much ppl will eat or drink but you certainly dont want to run out of food and you dont want to waste your money by getting too much. another good idea is looking up a beverage recipe online and seeing how many ppl it will serve and weighing that against the volume of the beverage ex.
http://www.foodnetwork.com/recipes/alton-brown/cape-fear-punch-recipe/index.html