Food amounts for catering depend on the type of gathering, the time of day and the type of food. The amount of food you prepare should also depend on the budget. You would approach an event differently when clients indicate that money is no object and they want abundant food, than you would when customers communicate that they are on a tight budget and want you to save them money.
Think in terms of one and a half pounds of food per person as a starting place. In most situations this is more than enough food, so don't worry if you end up actually making a little less. Divide this amount among the various dishes you will be making, taking into account whether someone else will be preparing some of the food. Assess which menu items people are likely to eat in the largest quantities. People generally take more of the main dishes and less of the side dishes.
Consider the time of day when the event will be taking place, as well as the time of year. People tend to eat more at meal times than at events taking place between meals, and they eat less at lunch than at dinner. They will eat less at an early lunch or dinner than at a later one. They will also eat less if it is a very hot day.
Consider the type of event you will be catering. People tend to eat more when they feel comfortable, such as at gatherings of friends where the guests know each other well. They eat less when they are trying to impress one another, such as at business meetings. Ask your client about variables that may affect the quantity of food you will prepare.
Figure at least two units per person for finger food items served as discrete units (such as stuffed grape leaves). Prepare more if there will be a limited number of appetizers, and less if there will be quite a few.
Figure about an ounce of total cheese per person for cheese plates. If you are catering for 80 people, and you are providing 5 different kinds of cheese, you will only need a pound of each in total (454gr. X 2.27 Kilogram = 2268gr), unless the particular group is especially fond of cheese or there isn't much other food.