I have found that no matter how many shrimp you produce you are probably run out. Three per person? I bet some people will eat twice that much. One way to control the consumption would be to pass the shrimp instead of on a buffet. That way you have a more calculated idea of how many to prep. If it is buffet or nothing, we always say-"I have figured on X amount of shrimp per person, and it is likely it will all be eaten,if you would like more than that this is what it will cost." That way it is up to the client. And it is always clear in their head if they see an empty platter that they got their moneys worth.