The answer is that there is no way to find out. Not all martial arts schools that offer or promote to black belt, record and report this information. Even those that do, do not all report to the same institutions or organizations. To further complicate the question, many organizations do not consider others to be credible and do not recognize their ranking system and practitioners. To add another layer to the onion, anyone can go into any Martial Arts supply store and purchase any belt they want and go about proclaiming to be any rank they choose. Another layer, the many varied styles and types of martial arts.
This question simply cannot be answered with any degree of accuracy or accepted results.
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In this expert's opinion, I would say about 1%. The rest just think they do.
Actually, there would be no way to determine an accurate answer to this since there are a large number of people who would claim they know "some" yet might have only taken a few lessons, or watched enough Martial Art on television, movies, or YouTube to become "experts."
There would also be the difficulty of defining a Martial Art. Some people believe sports like boxing, or Wrestling, cage fighting, MMA are Martial Arts while others don't. You would also have to consider all the former students as well as active practitioners.
Since each recognized Martial Art system is probably represented by a number of different established organizations as well as independent schools, and none of them publish complete records, nor compile a master list shared between all organizations and systems, no one really knows.
It is a very small percentage. Most Chinese martial artists practice some form of Kung Fu.
Karate is from Okinawa and is a blend of some Kung Fu and Okinawa wrestling.