The simple answer is about 2-6 drinks per person. When considering how much alcohol to buy for your party, here are the main factors:
Guest primary age group
Hours of people drinking
Type of drinks you want to serve
Guest Primary Age Group:
Age:20-30's--- 4-6 drinks
Okay these guests might cost you a small fortune, but there are still ways to save. Just like we used to be (or are now) the twenty somethings can put a serious dent in our pocket book. Not to worry, if you limit the selection of alcohol, and stick to domestic on beers everything will be fine. Make sure you have enough beer to supply an army, some midgrade wine for the ladies, and 1 or 2 liquors for shots.
Age: 30-40 --- 3-4 drinks
A little more refined, they are not concered with getting their "buzz on" as they used to be. I would spring for some decent wine, decent beer (like stella) and a small variety of alcohol for mixed drinks. The main bottles of alcohol you should make sure you have on hand are Vodka, Bourbon, and Scotch.
Age: 40+. --- 2-3 drinks
These are the people more worried about getting up in the morning above all else. If it is a really special occasion, then you may want to consider the advice of the 20's. Generally you should have a couple bottles of single malt scotch and some nice bottles of red to get things started. Beer is not as much as concern, so focus on a few bottles of good alcohol and everything should be fine.
The key is always limiting the choices. Too much variety gets expensive and messy in your home or venue.
Hours of the party.
People will generally show up about an hour after it begins. (sometimes later) Consider your invitation hours the consumption time for drinks. Always have a solid end time, because there is nothing worse than wanting to call it a night when "drinkers" are just getting started. Most calculations are based on 4-5 hours of open bar. If longer that 6, start considering adding a small percentage of the overall total to your alcohol buy list.
If the occasion revolves around drinking, (weddings and birthdays) you may want to take this in consideration during your purchase. Every group is different, so the amounts and types will vary. For instance, if you have a group of friends who loves Crown, then I would certainly pick up a couple of extra bottles, and reduce the number of bottles on the Vodka. The groups will sometimes deviate from the listed alcohol above, but that is for your to determine. Use our calculator and alcohol guide to finalize everything.