Although this recipe calls for 2 pounds of fish it can probably easily do twice that amount if fish. Feel free to halve the recipe if you like. I think I'd still use the whole egg, but maybe cut the soda water or beer back a little bit to compensate. You can add a little more of the soda water/beer if it the batter seems too thick. Let the consistency of the final better be your guide.
In response to a question about batter sticking to the bottom of the deep fryer basket, I've added this note. Thanks to Rob for reminding me to add this tip.
I use the same trick they use in the fish and chips shops. Don’t just drop the fish immediately into the oil, take a few seconds with each piece. Get a good grasp on one corner of the piece and almost submerge it up to the point where the tongs or your fingers are hold in the piece of fish. Being very careful not to burn yourself, sway the fish back and forth in the oil for about 5-10 seconds before dropping it completely into the il. This gives it a few seconds for the outside crust to set and it wont be able to seep into the metal grating that forms the basket.