Many advocacy groups (particularly those making claims about human trafficking) fudge their numbers to make their cause seem more important, and you can see why, we would probably not be discussing this article without such an eye catching statistic in it.

If you try to see how they got this number in their report it is "credible estimates/sources" that are "on file at OHCR". So there's no way to independently verify this, you have to just trust them, which doesn't breed confidence in their estimates.