I think in many dishes you can add quite a lot, but it blends and cooks in with other ingredients so that the "fish sauce flavor" does not jump out as such.

I make mapo tofu with 1 tsp each of fish sauce, oyster sauce and light soy sauce. I don't think anyone would think it tastes like fish or oyster sauce in any way, but it doesn't taste right at all without them. The same goes for many other dishes.