It depends on how much your guests like etouffee. A serving size of 3/4 cup is a good starting point.  That should be enough for most diners to enjoy the dish without feeling overly full.  If you're expecting heartier appetites, you may want to bump up the portion size to 1 cup per person.  If you really want to go all out, some restaurants serve up to 2 cups of etouffee per person.  Additionally, consider any side dishes or other accompaniments that will be served alongside the etouffee.