Everyone always wants numbers for the maximum or minimum of fish, nitrate, pH, GH, etc, etc.. But the fact with nitrate is that it should be as close to zero as you can manage to keep it. No mention is made of the fish species or water parameters (GH, KH, pH and temperature are parameters; ammonia, nitrite and nitrate are conditions). Having 5-10 ppm nitrate in the source water is not the end of keeping fish, but it does mean you need to be even more vigilant as the normal biological processes in any aquarium will cause organics to accumulate and nitrates tend to increase. And you do not want them higher than they already are in the source water.
Regular (once a week) substantial (50-60%) water changes, assuming the parameters of tank water and tap water are basically the same, help to keep nitrate low. Not overcrowding, not overfeeding, keeping the filter well cleaned, the open areas of the substrate clean, also help. Aquatic plants especially fast growing species help (they remove ammonia which means less nitrite and thus less nitrate).