That's a good question. I'd kinda like to know myself, but if I had to just shoot in the dark, I'd guess 10-12 smallish fish max. Most everything else (corals, inverts, etc) will have such a small impact on your system, you probably don't have to worry about them too much. You can do larger amounts without overloading depending on your equipment, though. I think if you skim heavily and have plenty of water movement/co2 exchange, you can do more.
Any "rule of thumbs" out there?