Who told us that? Specifically, whose PoV? Unless it was in Rand's, the PoV character who gives us that information has no business knowing how many there are, and so the information is suspect. The most likely sources for that datum are Elayne or Toveine, either of whom the Asha'man would be more likely to hide their true numbers from than the reverse. Toveine & Gabrelle came to the Black Tower to gentle & kill every man there, and even as much of a douche & an ass & a p-whipped weasel as Logain is, I can't see him spilling such information to his pets casually. For all we know they have been deliberately left in the dark, to prevent the Tower from getting them all. The same reasoning would go with Elayne, what with her all but being raised by a Red. Of all the male channelers in the world, ONLY Rand would have the slightest reason to trust her, and he had not been in contact with her.

Rand himself, in KoD, tells Logain that the BT nearly equals the WT, IIRC. I would take that as a far more reliable piece of information. Especially with it being so much more current and all.