I'd say it's fairly rare. Most therapists often won't diagnose patients, because it's considered not helpful to the trust relationship and trying to make someone that's narcissistic understand that they are narcissistic is... Hard, because of the splitting and the disconnect between the internal self image and the external presentation.

They'll often mirror or turn it back on you and there's not much you can say at that point ("I'm a narcissist? No! You are a narcissist!"). It's a tricky disorder.