This number includes the amount needed for chilling beverages. If you're going to buy too much of anything, buy too much ice. Protect your home. Just because your instinct is to look for a coaster when you put a wet drink down on someone's table, that does not mean everyone else does. The appetizer rule.
Caterers assume people consume eight appetizers each.
This is a good number to stick to but take into account how many hours you're serving cocktails and the type of appetizers you're serving, and realize you can bring the number and the price down considerably by serving cheese and crackers, chips and dip, and vegetables. Any food item will look even better with just a few minutes of design. If you plan to serve food platters, decorate with small flowers, grapes, brightly colored napkins, etc. If you're hosting an evening event, remember that few decorations spruce up a space as inexpensively as candles. Minimize oven time.