I have been asked several times by friends about how much alcohol one should buy to produce an open bar for say a 100 person party or wedding. It's really hard to say, because you can't really tell what the crowd is going to want to drink. But being in the industry for over 16 years I have come to see what bartenders and caterers have told me about my open bars.
Below is my best estimate on what one should buy for their open bar without the help of a caterer. The purchasing of these amounts for a reception has not failed me so far.
Beer: domestic, imported, and light beer - 3 dozen of each.
Wine: 3 dozen white, 4 dozen red (5-8 dozen total)
Champagne: 1 dozen bottles (only fill up half for toasts) also you may just want to do toasts with what people have in hand. This saves a lot of money and you can just buy 6 bottles of champagne for champagne cocktails at the bar.
Vodka: 5 liters
Gin: 3 liters
Tequila: 2 liters
Rum: 2 liters
Whiskey: 1 liter
Scotch: 2 liters
Bourbon: 2 liters
Dry Vermouth: 750 milliliters
Sweet Vermouth: 750 milliliters
Kahlua: 3 liters
Bailey's Irish Cream: 1 liter
Grand Marnier: 1 liter
Triple Sec: 1 liter
Amaretto: 1 liter
Sambucca: 1 liter
Brandy: 1 liter
Frangelico: 1 liter
Chambord: 1 liter
When shopping for your liquor, make sure to work with a local liquor store and ask about their return policy. Some liquor stores will give you refunds for unopened/unused spirits. Beer is the exception, most will not take back beer.
Buying your own alcohol for your reception can save loads of money. Ask your caterer about their policy of serving self bought liquor. Most caterers are cool with it...but there are some that will charge you more than if they served and bought it themselves.
Hope that helps answer your liquor questions.
Photo from Blender Artists