A cocktail hour is two planning problems that have to agree with each other: the bar and the bites. For drinks, the rule is one per drinking guest per hour, weighted toward whatever fits the crowd, and the share who actually drink is rarely everyone. For food, the caterer rule is five pieces per guest in the first hour and three each hour after, and the single biggest question is whether the appetizers are the meal or a prelude to dinner, because that doubles or halves the count.
Plan the two together so they balance: more drinking calls for more substantial bites, and a longer event needs both. Use the drink calculator for the bottle, can, and ice counts, the appetizer calculator for the per-item piece counts, and remember the things people forget, namely ice at over a pound per guest and a real option for the non-drinkers.