I could not find any reliable information on the prevalence of scalp acne on Google...

Maybe you are confusing acne vulgaris with folliculitis or dermatitis?

You can find information about it even on acne.org, http://www.acne.org/scalp-acne.html

The only reference on that article is http://www.dermnetnz.org/acne/scalp-folliculitis.html

I can't take this anymore, head hurts.. Just pretend that scalp acne don't exist then.