>>2
Partying isn't a question of numbers but of quality of people. Starting around 100 you don't get to do stuff with everybody anyway. I prefer having an awesome night with a few close friends and crazy people than an average one with the same quantity of "filler" people added.

I agree on the rest, except alcohol quantities but lots of heavy drinkers in my circle, so... in general we count at least 4 beers (not light shit) and a third liter of strong alcohol per person. Ie, for 3 persons, count a 12 pack and a bottle of vodka minimum.

The easiest thing to do is have everyone bring a bottle of hard alcohol or a pack of beer. Try to get different stuff from each people too. If there are remains, you'll keep then for the next party. Have one or two of them not buy alcohol but instead the same amount of money in chips, peanuts, and whatever munchies you might found.