If it is per head, get a minimum number guaranteed, plus an agreed upon price per extra and in writing the number of extra they want you to prepare for.
Be sure that it is written that this is a per person fee and that the amounts you bring to cover the extra, which are not used, are YOURS not theirs afterwards.

If it is an agreement for a fixed amount of food for a certain cost, get the amounts of food you are providing in writing. If they want you to prepare for MORE, the contract should specify your responsiblity (for example, will make available up to x additional trays or plates at $y per tray/plate). Again, the two issues are, running out of food, and the employer thinking that they have paid for food and get to keep it, when they have not paid for.

That said, if this is self service buffet, you will use the most food, especially if, as is likely, alcohol is served- for 400 you will want about 160 entree servings of each entree. Only about 140-150 if there is a server for the meats.