Where does it pass from true self-confidence to the point where someone has narcissism?

Different dimemsions. Narcisssm has to do with a personality style that over-estimates one sense of power and importance and uniqueness. It is not necessarily accompanied by true self-confidence, because often such individuals recurrently find themselves not living up to their dreams and expectations--almost always because of actions of others, of course. True self-confidence is based on past successes.
Narcissism. The hallmark of a narcissist is an immense ego, grandiosity, and self-importance, as well as being entitled, and a tendency to use and objectify others as well as thinking of people in all-good or all-bad terms instead of shades of gray. The core of a narcissist is profound insecurity. Self confidence is about being good enough. Narcissism is about being the best at everything in the world.