The amount of sodium a softener adds to your water will depend on how hard the water is.
As an estimate, you can multiply the amount of hardness by 2 to get the approximate amount of sodium added to an 8oz glass of water.
So, if your water is 15 grains hard, your softener will add 30 grains of sodium to a glass. 1,500 grams of sodium per day is what is recommended for a healthy adult.
Even if the amount of added sodium is relatively low, many people choose to install a reverse osmosis drinking water system (which will remove sodium) to improve the taste.