Throwing out a big party is not a trivial task. The first tough thing is to lock down the number guests. And after you finally managed to get the number, the next tough thing is to figure out how much food and beverages you should prepare.
On one hand, you can always prepare more to make sure you are not embarrassed for not enough food or drinks to serve every guest. But on the other hand, a lot of left over is a big waste. This chart from Heather at Chick A Bug is good reference guide. Heather must be a party master. Give the guide a try for your next party and let us know how it works.