It's always better to order too much food than not enough. You can always keep leftovers and if you do not have enough food, that can bring a halt to the party. But since you may not want to waste money on food, you should try to estimate how much food you will need as closely as possible. There are several questions you should ask when deciding on how much food you need.  

Will Your Guests be Hungry?

Begin by understanding how hungry your guests will be. The earlier in the day, the less likely that they will expect to be fed. Also, if there is a reason to believe that they will have already eaten, you may decide to order less.

Should Your Guests Bring Food?

Asking the guests to bring food can help minimize the risk that you won't have enough, as long as they take their leftovers home. However, if you do not have enough space at the scene of the party or no way to refrigerate the food, you may not have this as an option.

How Many Appetizers Do You Need?

Appetizers can complicate how much food you should offer. If there is a lot of time between a party, you will need to offer more appetizers. But by filling up on appetizers, you will also want to make the meals smaller so guests that are full won't have to throw away as much food. If you aren't sure of how full your guests will be, set-up a buffet so it is easier to keep the leftovers. Also, make enough appetizers so that guests can snack on them after dinner. Also, try to guess which appetizers your guests will prefer, such as shellfish, and order more of these.

How Well Will Your Food Keep?

The more food you serve that keeps well, the less you will need to order a lot of food. For example, meat products tend to not last as long, while salted food will last longer. You may also be able to freeze some of your dishes, such as soup, to be thawed at a later date.

Will You Serve Alcohol?

If you are not serving alcohol, you will likely need about four gallons for every 25 guests. If you are serving alcoholic drinks, expect to serve one ½ drinks per guest every hour. Fortunately, most beverages that aren't consumed can easily be kept for a later date.

Should You Order More Food?

While it is difficult to predict how much food you should order, there is fortunately always a backup. You can always order food from a food delivery service (such as Nitza's Pizza - The Original pizza) so that you'll never run out.