How much food to serve at buffet for 150 people?

Well, you have 450 guests, so you will need 450 appetizers. Most guests however will eat more than one, so a safe bet is to triple the number of guests, in this case 1,350. It depends on how you are serving however. If this is a sit-down meal with service, go back to one per guest. If this is more of a cocktail party venue, look more at that larger number, and perhaps as many as 2,000.