Many people have a real name in their account because their account is linked to a social media or Github account, so the impact of this would be rather large. I, personally, always use my Github account to log in to Discourse forums as that reduces the need for creating yet another account with another (or the same) password, and Github has my real name.
E-mails from the forum are already sent using the real name in the sender field, which gives you an indication of the amount of people having a real name.