Start by considering the who, when, and what of your party.
Who's coming to the party and how formal is it? Young people seem to eat more than an older crowd will, and guests are likely to nibble more at casual parties than at more formal functions.
When is the party? Appetites will be greater if it's close to dinnertime than if it's late evening. Also, people tend to eat more when the weather is cold. People will eat less when a meal is soon to follow. If a meal will be served shortly after the appetizers, allow 4 or 5 hors d'oeuvres per guest. If a late meal is planned, figure 6 or 7 per guest. When appetizers are the meal, plan 12 per guest.
What are you serving? Guests will eat fewer heavy hors d'oeuvres than they will light ones. Hot appetizers tend to go faster than cold ones. Plan on each person sampling one or two of each type, except for shrimp -- allow plenty of this popular offering.