The PURE study out of McMaster University, along with several other recent studies, has shown that maximal health is at 5 g of sodium a day, which translates to 2-1/2 teaspons of table salt (and includes salt already in the food as well as added salt). People on a ketogenic diet could probably stand a bit more, but I have no idea how much.
Five grams of sodium is the sweet spot in the risk curve; go below or go above that level, and risk of cardiovascular disease and death increases. The risk from higher levels of salt increases more slowly than the risk of not getting enough, except for people with high blood pressure, for whom the risk curve is identical on both sides of the sweet spot.
Here is the principal investigator discussing the results of the study at Low Carb Breckenridge 2018: