I get asked this very question all the time! It is one of the tricky ones to answer though, as it depends on so many different variables. Is the wedding or event during the day, or late in the evening? Are you hosting a full bar, or only beer and wine? Is there a house or signature cocktail(s) being offered? Do you have a heavy drinking crowd or not? How long is the cocktail hour going to be?
All of these questions can alter how much you should plan on purchasing for your event. Some people just give the standard answer of a bottle of wine per person. That really doesn't help much, as that may be true if you are only hosting beer and wine, but if you are offering a full bar to be hosted for the duration of the night, that sometimes would not be the correct amount. My best advice, and it may sound too obvious, but run the drink calculator on Evite. The best thing about it is you can plug in all the facts; how many guests, how many hours drinking will be taking place, what you are serving, and how the crowd is in terms of heavy, light and non-drinkers. It will then calculate what you should purchase, and it usually is pretty close to what I would have come up with after a few standard calculations. As a heads up though, the beer amount tends to come out about 10-20% higher then what I would typically recommend, but if you know you have a lot of beer drinkers or it will be at a very warm location, it can't hurt to have enough cold beers on site.
No matter what you do, know that it is always better to over estimate then to run out of beverages at your wedding! I can't even tell you how many times I've sent my assistant to the liquor store (if there is one nearby) to pick up more drinks. Or, if you are serving a house cocktail, make sure you have all the right mixers and garnishes. Also check with your local stores (BevMo, Costco, etc.) on their policies on having them buy back unused cases or bottles, and ALWAYS make sure you have a designated person to take home your leftovers. It can be a lot and usually won't fit in your getaway car, and can easily be used the next day at your day after brunch or BBQ.