From "The Idiots Guide to Mixing Drinks"
Assume a 4 hour duration for the party:
You will want 1-litre bottle of you "front bar" liquour for every six
guests. That is at least Vodka, Gin, Bourbon, Scotch (you may choose to add
Rum and Tequila).
1 bottle ea. of sweet and dry vermouth
1 case of beer for every 10 guests (a keg serves about 70 if you think
that will be cheaper)
wine consumption can vary. 1 case (12 bottles) contains about 60
servings.
the mixers depends on the variety of drinks you want available. But
figure 2 quarts of mixer for every litre of light alohol.
garnishes - 1 lime for every 5 guests, 1 orange every 25, 1 lemon every
50, 1 jar ea. of olives, cocktail onions, and marachino cherries.
don't forget to provide snack foods (naturally) and non-alcoholic
options.