It is easy to estimate how much food can be consumed by some 2-4 people, but what is these 4 people become 20, 30, 40, 50 or more? If you are on a catering business, this is perhaps among the things you should be more adept to calculate. If there is a surplus and the client has agreed to pay for it, the better. However, it would be a sure dilemma if the foods have run out way before the end of the event.
You need to determine from your client the number of people expected to attend the occasion, the time duration of the party, what foods are preferred to be served by the client and when each dish shall be served. To be on a safe ground in determining how much food to prepare, it is better to add a little extra servings in foods and drinks.
For the drinks and beverages, assume that each person will drink three servings. Coffee for instance is demanded during mornings, late afternoons and after meals. Water is essential and is greatly demanded especially on hot and summer days. Keep in mind to prepare creamer and sugar substitutes.