Basically what the title says - it becomes clear in the books that a channeler can be turned by 13 myrdraals and 13 channelers working in tandem. Is any reason given in the books for why this wasn't used far more than it was.
The wiki list only 14 people we know have been turned, although it is likely more were turned at the Black Tower.
What makes me wonder the most is that the Black Ajah has been around since at least the Trolloc Wars, but doesn't seem to have practiced Turning. As evidenced by the hunt for the Black Ajah under Seaine Sedai, it wasn't that hard to abduct a Sister for an evening, and the strategic implications of being able to slowly turn the White Tower black seems immense.
This was just an idea which occured to me while rereading the series, and I am not that far into it yet, so there may be some obvious reason I've missed. I don't seem to find much discussion of this by either a quick forum or google search so it kinda seems likely, so if someone here with a better grasp on the lore than me could weigh in, it would be much appreciated!