Even just a casual search on Korea's biggest online map service Naver Map shows easily a dozen massage parlors in a single neighborhood of Seoul. There is no accurate data, but some argue that there are more than 100,000 such massage businesses nationwide.
Most of them are illegal.
No, I am not talking about places that provide a 'happy ending'. I mean everything from the massive Foot Shop franchise with locations all around the capital, countless Thai massage shops, to places that have the phrase "sport massage" in the name. They are all breaking the law.