In May of 2012 we find a blog post from Ming plugging his $800 email mentorship program. At that time he'd had more than 150 students. The program was open ended. Take all the time you like.
Yesterday or thereabouts we see a new plug. Same price. A one year limit has been introduced. And more than 200 students in total, with 50 still working through it.
If I read this right, in the last three years and change he's signed up about 50 new students and currently has 50 students in the pipeline. No wonder he's introduced the one year limit.
Had he been running this program for 15 years? Was there a huge pop early on? Does he just have a long tail of foot dragging students signed up from before the one year limit?
All these things are possible. But they're not the most likely explanation!