Cooking temperature and time adjustments for cooking several whole fish in a conventional oven?
That depends on how large your oven is.  I certainly couldn't do 6 whole fish in mine, but then I have a 24" wide oven.  If you have a 36", it would be completely possible (these are standard US sizes; no idea what the equivalent would be in other countries).

Your main adjustment will be dealing with the extra mass to be heated by the additional fish, which will cause the oven to take some additional time to reheat.  Again, this depends on how hot (BTU) your oven is; a hot oven would overcome the extra mass in a few minutes, but an underpowered (for this purpose, anyway) oven might take an additional 15 or more minutes, which would not be good for the texture of the fish.

So, It Depends.  If you have a fairly powerful, large, high-end stove, I'd be ready to flip them at 10 minutes.  Less powerful ovens will take longer.

Also, if the fish are individually closer together than 1", you'll have the issue that the edges of each fish will be soggy from the moisture of the fish next to it, and they won't get good texture.  If they're touching even slightly, they will stick together.  You'll also want to try to ensure some drainage in the pan, lest all of the liquid runoff collect under one fish and make it slimy.