I was the elder sibling and my little brother was absolutely tasked with keeping me in line.

And yes, there's absolutely no way to get accurate numbers because not only are NPDs notoriously treatment resistant, but most of them would never consider therapy in the first place unless it was to learn the language to use against other people.

And believe me, they do. I've had many narcissists in my life, and every single one of them used what I learn in therapy against me.