Pick a number
When cheese is one part of a meal, buy about 30 grams of each cheese for each person. Three cheeses come to about 90 grams a person. When the board is the meal, buy about 100 grams a person.
Buy the amount you will eat
Ask for a wedge you will finish in a day or two. A fresh cheese spoils sooner than a hard wedge, so buy less of the fresh one. A few small pieces beat one large piece of a single cheese.
Leave room for the rest
Bread, fruit, and nuts fill the plate. If the table already has a main dish, cut the cheese amount down. A hard cheese goes further than a soft one, because a thin slice still tastes clear.