so going in this rule and understanding the purpose of adding sodium to water then it must be clear that :
The amount of sodium added to bottled water depends on the equivalent water classification during the softening process. For soft water, 0-25 milligrams is added per liter, moderately hard water 25-50 milligrams sodium per liter, hard water 50-75 milligrams per liter, while more than 75 milligrams of water is added to very hard water.
so the addition of sodium is linked to the softness of water if it is soft water sodium is low needed and if hard the sodium is more
However, water softeners are installed to provide water for washing, not drinking, and most people prefer to drink hard water because it tastes better.