A typical 750ml bottle of champagne fills three to four glasses. For small glasses, four servings per bottle is a safe estimate. For larger glasses, estimate three servings per bottle. So, if you expect 75 guests and plan to serve each guest one glass of champagne, plan to purchase 19 to 25 bottles. Keep in mind those guests who are under the legal drinking age as that will affect the number of bottles you need.

(Source: zazzle.com)