Caterers- Who as a group, it should be noted, love parties- work on the formula of one bottle per person. That figure is based on the assumption that it is better to have unopened bottles of wine leftover than it is to run out. (It’s also based on the assumption that you’ve got plenty of designated drivers.) Since it is impossible to predict precisely how much (or how little) each guest may drink, a bottle per person allows for quite a comfortable margin of error. If the party will take place over several hours and is especially celebratory (like a wedding), and if you know everyone will be having wine, you should plan for somewhat more. One bottle of wine will yield about five standard glasses of wine when each glass is filled half-way.