I was asked to provide desserts for a Christmas party for 400 people in attendance should I charge per person or should I charge per dessert Help!

Depends on what they are asking of you?  One large cake for 400; cupcakes for 400; a variety of dessert options to feed 400.

If it is cake or cupcakes, I would price per serving.  If it is a variety of desserts, I would price per serving per dessert - do they want 5 cheesecakes, 4 pecan pies, 5 apple pies, and 6 dozen cream puffs?  Each of these would be priced differently as they have a variety of ingredients and labor required.

DON'T SHORTCHANGE YOURSELF!  This is going to be a LOT of time and effort. 

@KimmyO the choice is up to me they just want desserts that can be self served.  

I'd charge per dessert rather than per person as numbers might change but depending on what you do you can't simply deduct one serving. For example if you make cupcakes, you'd make them in dozens or even for a tiered cake or cookies you'll have difficulties deducting a single serving...