Amount? Depends on how big the salt tank is. Most I've seen will hold numerous 40lb. bags. Depends on how often you want to fill it up too. So you don't run out I would fill it (4 or five bags is probably about right) then monitor the usage and set up a schedule as to when you need to reload. The Sears unit I have now has a salt monitor on it that blinks at you when the salt gets so low. Good luck.