We generally recommend the following rule of thumb: 1.5 glasses per person for the first hour and 1 glass per hour for every hour after. For example, if there will be (100) guests at a 3-hour cocktail party, you would order (1.5 x 100) + (1 x 100) + (1 x 100) = (350) glasses. 

In order to best determine the quantities and style of glasses you'll need, consider the amount and types of beverages you will be serving in addition to your guests' preferences.