Also, it somewhat depends on what you mean by "frying". If you are dipping them in beer batter, or tempura batter and frying, you won't need as much fish. If on the other hand, you are just dredging them in flour, frying and lightly salting them (that's how we fried fresh caught brook trout, yum) then you will need more fish as there is no bread to fill up the belly. The same is true if you are baking, poaching, broiling, etc.
But as most people tend to prefer their fish coated in beer or tempura batter, figure in the coating.
Another thing you could do, an one that would extend the fish even further, is to cut the fish into cubes before dipping into the batter. Then serve as "popcorn fish", or even as fish fingers. You could add a little cayenne to the some of the batter, some taragon to some, and leave plain for the rest. Then you offer a variety to your guests. Mkae wure to mix ketchup and powdered mustard for one sauce, ketchup and horseradish for a second, hoey and mustard for a third, and mayo, taragon,, dill, and pickle relish to make tartar sauce. Plain kethup and ordinary malt vinegar work too.
Good luck with your shindig. I hope it goes well for you.
And as far as economical fish go, pollock is great. It is mild and slightly sweet, and is good with all of the above cooking methods.
Seeeeeya; Goodweed of the North