As with any criminal enterprise, human trafficking is difficult to measure, since the participants do not readily provide statistics.  The best estimates, however, are summed up well in a study by the Renewal Forum, a Washington D.C. based lobby focused on issues of human dignity, in particular the commercial sexual exploitation of juveniles.  Click on the following title to see a pdf of their estimates of how many juveniles are victimized by sex trafficking:

PDF Link:  HOW MANY JUVENILE VICTIMS OF HUMAN TRAFFICKING ARE THERE?