It only takes a very small pinch, speaking from experience. Like less than an eighth of a teaspoon will almost eliminate the bitterness in a large mug of coffee, without adding a salty taste.
At least to my tastes, but then I'll literally eat coffee beans so my scale for that may be a bit skewed, though if you drink black coffee already you're probably similarly acclimated to it. It wipes out that sort of sour-bitterness coffee can have, which is the specific thing I find unpleasant in black coffee myself.
tbf, I'll give it a try. That being said I do like the bitter/astringent notes in my tea and I steep it for ages to draw them out, and I don't drink with sugar, so adding salt seems like a preference/flavour thing more than an advice thing.
Edit: also it seems that where I live has soft water, which apparently skews brews towards bitterness, so adding salt kinda makes sense if you want to mellow it out. I wonder if adding salt after brewing with hard water is less necessary?