The average rule in estimating number of drinks for an event is 2 drinks per guest for first hour plus one drink every hour after.
I recommend using BevMo’s drinking calculator to estimate the number of drinks for your event. Make the estimate more accurate by only entering the number of alcoholic-drinking guests. Also, don’t forget about the champagne toast. That’s roughly 5 drinks per champagne bottle. And increase the numbers by 25% if your guests are heavy drinkers and decrease 15% for afternoon events. The same average rule applies for non-alcoholic beverages.