CBC ran a documentary on this very issue, (trafficking of women for forced prostitution). They didn't talk in total numbers but it was clearly in the tens of thousands without a question. The girls get told they are offered jobs in foreign countries and then wind up being forced into prostitution.
Part of the problem is that the reality doesn't fit the image. We think of what you see on US Network TV where the girls spend their days in dimly lit buildings under constant guard and beatings. The truth is that they are usually blackmailed, their passports are withheld from them, and they are basically just kept isolated. They usually end up going home eventually. There was a ring busted here last December after some John became suspicious and tipped off the cops. They were operating out of one of those cheap motels. So it happens everywhere.