The percent of the general population that gets Chronic Traumatic Encephalopathy? This isn't a naturally occurring phenomena. It's the cumulative result of repetitive skull impacts and concussions.

Google gives 3-6% in non-athletes

Basically zero. It only happens if you keep getting hit in the head.

A recent prior study - I think one of those mentioned in the article - said other studies found very little, maybe one or two brains in the samples. And that included a study of military veterans.