Whichever of these two figures you choose, there are still a lot of slaves in the world today:

According to sociologist Kevin Bales, who founded and directs the new abolition group Free the Slaves, an estimated 27 million people are enslaved around the world today—more than were ever enslaved at any single time in history. The United Nation’s International Labour Organization estimates are a more modest 12.3 million—which is still a shocking number of people forced to labor against their will, unable to walk away, for no compensation. Much of the reporting on this phenomenon has been on women forced to work in the sex trades. But the U.S. State Department reports that many more people are enslaved in far more ordinary endeavors: mining coltrane, growing cotton, domestic servitude, and fishing in the south Pacific.

This is not just a historical problem: this is a major issue around the world. Bales has written a number of book on the subject and these would be a good place to start in learning more.