Emphasis mine:

Question: In Winters Heart, you mention that back in the Age of Legends, there were several other Forsaken that the Dark One had killed because he suspected they would betray him. What's their story?

Robert Jordan Answers: Forsaken was the name given to Aes Sedai who went over to the Shadow in the War of the Shadow at the end of the Age of Legends, though of course, they called themselves the Chosen, and despite the tales of the "current” Age, there were many more than a few of them. Since they occupied all sorts of levels, you might say that many were equivalent to some of the lesser Dreadlords, but it would be incorrect to call them so. At the time, they were all Forsaken—or Chosen—from the greatest to the least.

Some of those Forsaken the Dark One killed were every bit as high-ranking as the thirteen who were remembered, and who you might say constituted a large part of the Dark One's General Staff at the time of the sealing. With the Forsaken, where treachery and backstabbing were an acceptable way of getting ahead, the turnover in the upper ranks was fairly high, though Ishamael, Demandred, Lanfear, Graendal, Semirhage, and later Sammael, were always at the top end of the pyramid. They were very skilled at personal survival, politically and physically.

In large part the thirteen were remembered because they were trapped at Shayol Ghul, and so their names became part of that story, though it turned out that details of them, stories of them, survived wide-spread knowledge of the tale of the actual sealing itself. Just that they had been sealed away. Other Forsaken were left behind, so to speak, free but in a world that was rapidly sliding down the tube. The men eventually went mad and died from the same taint that killed off the other male Aes Sedai. They had no access to the Dark One's protective filters. The women died, too, though from age or in battle or from natural disasters created by insane male Aes Sedai or from diseases that could no longer be controlled because civilization itself had been destroyed and access to those who were skilled in Healing was all but gone. And soon after their deaths, their names were forgotten, except for what might possibly be discovered in some ancient manuscript fragment that survived the Breaking. A bleak story of people who deserved no better, and not worth telling in any detail.

- TOR Question of the Week