I think no one else has mentioned this, so I'll just pop in to say that I read Fray and as far as memory serves, I don't believe it said anything regarding the formerly large number of activated Potentials (now Slayers) and why at the time of Fray the world was back down to one Slayer. Or I should say, as far as the viewpoint of the comic was concerned, all we could see was that there was one Slayer. That could have been misleading.

So sorry, but no help.

I have the sneaking suspicion that Joss wrote Fray before he had the bright idea of activating all those Potentials. I'm not entirely sure of the timeline, but I think Fray came about pre-S7 or at the very least, concurrently. So has even he figured it all out? Only the Shadow (Joss) knows...