Six ounces per person is a bit light. I would figure 3 3oz. pieces as a serving. 25# or a bit more should get you what you need and a little extra. Being for church, you should know the crowd a bit... if they all like to eat like I do, I would get 30# (or more) of fish.
On a side note, don't forget the malt vinegar!
18.5 WSM, Green Weber Genesis320, Cast iron firepit(w/grill), and a Copper Weber 22" OTS