From what I have seen, both quanity and percentage is greater in taekwondo. This would be true if were any other art. Once you have the quanity, the percentage goes up because you now have a commercial dynamic in motion that perpetuates itself throughout the art.Since TKD is the most popular you definetly see it most rampant in the belt mills. What would be an interesting study is to see if the percentage of TKD schools that are belt mills are the same or more than that of other arts.
In other words you may have 100 TKD schools and 60 of them are belt mills (60%) and you have 40 Karate schools and 24 of them are belt mills (60%) so it would be the same. There just happen to be more TKD schools than Karate.
Daniel