In Italian we say "use your feelings" (vai a sentimento).

Since all pots are different and the amount of salt depends on the amount of water (not of pasta), you just have to try a few times and get the hang of it. Generally speaking, in American terms, I'd say a couple tablespoons of coarse salt, less if you use fine salt.

Anyhow, the bottom line is, it's not a pinch of salt, it's a fist of salt. Don't skimp on salt in pasta water. If you are concerned about the amount, just know that almost all of the salt will remain in the pasta water. If you want to check how you did, taste your pasta a couple minutes before it's done, and add more salt if it's bland.