When does NavMesh get updated at runtime?

I want to use NavMesh but there are some moments I still am not knowledgeable about. If my pawns and other movable actors are set to not affect mesh does it mean that at runtime NavMesh won’t get updated dynamically? Is it a better practice that I keep dynamic Actors from affecting NavMesh? I suspect NavMesh having to update at runtime drains quite a bit of resources. And is it okay to have many NavMeshes in my level?