how many plants will die if watered with salt water?
But if you're talking about landscaping plants, there are many which are tolerant of seaspray. They do well along the seaside with salt water and spray. If that's what you're referring to, let me know and I can point you in the direction of some landscaping plants best suited for the seaside.
Otherwise, I wouldn't take a chance watering my houseplants with softened water. I lost *all* my fish, my birds, and most of my houseplants because of inappropriate water. And it has made *us* sick as well! Treat your houseplants as they were meant to survive in the wild and they'll reward you.
Salt water is used by the state of Oregon as a weed killer on the coast, at least when I lived there.
We do not understand the saying "salt of the earth" here. Salt kills most things....