Many events are large-scale affairs. Guest lists often include hundreds of invitees, even though not all of the guests attend. When you are providing the meal, calculating the amount of food necessary to feed so many people seems daunting, but it really isn't-- if you wisely plan a buffet style spread. You will be able to feed 350 guests easily -- with enough for everyone to have seconds!
Choose a menu type. Decide what kind of food you want to serve at the reception. Maybe you want a hearty meal with all the trimmings but have a budget that can only handle finger foods or light snacks. This decision alone will help you determine the amounts you will need to feed 350 people.
Determine how many of the guests are children. Young kids don't eat as much as adults. Figure this into your calculations.
Select foods that produce a large yield. For example, a single, large roasted bird, turkey or chicken, can feed dozens of people depending on how it is sliced. When served singly, small clustered fruits, like grapes and cherries, also feed several people. Crackers, cruciferous veggies and cauliflower all have high yields per single unit.
Buy by the pound, not in literal quantities. It is too overwhelming trying to decide how many baby carrots or grape tomatoes to purchase for 350 guests. Instead, research how many pounds of a given item can feed a certain amount of people and calculate your grocery list that way. For instance, a precisely cut turkey (that is, two legs, two wings, etc.) that weighs 18 to 20 pounds will feed about 12 to 14 people. But if you cut it all up in choppy slices without regard to whole pieces, you may be able to feed at least double the amount of people.
Count on serving a variety of starchy side dishes. Rice, roasted cubed potatoes, penne pasta are foods that fill people up quickly and take up a good amount of space on a dinner plate. Also,they are relatively inexpensive and you don't need to count individual quantities for purchase.
Plan to serve a variety of foods. Have appetizers, a salad or crudite, dinner and dessert. It is quite likely that each guest will not eat every item in your buffet spread. If you serve different foods, you do not have to buy 350 guest's worth of each. In total, figure that each person, minus children, can eat about 1 liter or about 2.5 pounds by the end of the reception.
Hire an experienced caterer. They are accustomed to serving large numbers of diners, and thus would know how to calculate how much food they would need to prepare your requested dishes. They may also have assistants on staff -- or know how to hire a few for the day -- to serve the food and manage the amount guests are served.