How much sodium is added to water during softening?
This is directly related to the hardness of the water. For every 100 mg/l of calcium carbonate hardness taken out, 46 mg/l of sodium go in. Add to this any sodium originally in the water and you have the total sodium in the water after softening.
To keep below the 200 mg/l threshold for sodium, you can soften water containing up to 435 mg/l of calcium carbonate hardness, depending on the original concentration of sodium in the water supply, which must also be allowed for.