The number of hors d'oeuvres you need often depends on how long the cocktail "hour" is, the time of day, and if a meal will also be served following the cocktail hour.
For a two hour party plan for:
w 10 -12 bites per person
w 20 bites per person if the hors d’oeuvres replace dinner (e.g. party starts at 7:00 pm)
When hors d’oeuvres are served before a meal plan for:
w 5-7 bites when served before a light meal
w 3-4 bites when served before a full dinner meal
People eat more during the first 90 minutes. After two hours consumption picks up again, especially if hors d’oeuvres replace dinner.
Young people eat more than an older crowd and guests eat more at casual parties than formal functions.
Remember to provide a variety of foods to please everyone. That means something for the vegetarian as well as the meat eater. Having both hot and cold foods also helps to keep everyone nibbling happily.
If hors d’oeuvres take the place of dinner, also include several bites of sweets per person.