Thanks everyone! This is a huge help.

People love the snacks & hors d'oeuvres more than the food! since they are eating them first I would plan on at least 8 - 10 per person and always put out some other items as fillers such as dips, chips, or other snacks.

Will there be other food served at the party, or is this an hors d'oeuvre-only affair? If there will be a light meal served, then 5-7 pieces per person is good, if a full meal, 3-4 is sufficient. If it's hors d'oeuvres, only, Daniel Bouloud recommends 6-8 pieces per person, per hour (http://www.foodandwine.com/articles/hot-hors-doeuvres/sidebars/1), but I've also seen recommendations of up to 10-12 pieces per person. I personally always err on the side of caution - there's nothing worse than sending guests home hungry!

Overall count, but I think, unless these are large hors d'oeuvres, the number should be more like 5-7.