You need to allow about a bottle per person of wine, 2/3rds white to 1/3 red as a start. If serving fizz, allow 5-6 glasses per bottle and work out how many glasses you want each person to have. Depends what else you're planning to provide in the way of spirits etc. My advice is to stick to wine and beer, with soft drinks. if you decide to have lots of spirits and stuff, get someone reliable to act as a barman, or you'll end up with very ****** punters, as everyone will think you're fab for providing expensive stuff and help themselves to it with impunity! Most outlets for wine will sell to you on a sale or return basis, provided lables are intact. Most decent outlets will also advise you on quantities. If you go down the sale or return route, and you are planning to chill the drinks, can I suggest a really good idea? Use a big dustbin and fill it with ice, water and a bag of ordinary table salt (which makes the cold water colder). Then sink your bottles into it in thick garden plastic bags (those ones you use for garden waste). this means that the bottles get chilled, but aren't in direct contact with the water and ice, so the glue on the labels doesn't dissolve and make the labels fall off. Have a good party!