While it is perfectly true that salt or rather the chloride in salt does partially protect a fish from some of the effects of nitrite poisoning.
Don't add more salt than is recommended.
Adding salt unnecessarily to a fresh water tank is never a good thing to do. It has no benefits and it makes some plants more difficult to grow and will even reduce the salts effectiveness as a treatment should it be required.
The answer is probably much lower than you expected, but it really doesn't take very much salt to protect fish against nitrite poisoning.