According to that National Geographic article titled “21st Century Slaves“:
There are an estimated 27 million men, women, and children in the world who are enslaved — physically confined or restrained and forced to work, or controlled through violence, or in some way treated as property.
Therefore, there are more slaves today than were seized from Africa in four centuries of the trans-Atlantic slave trade [11 million total, and about 450,000, or about 4% of the total, who were brought to the United States]. The modern commerce in humans rivals illegal drug trafficking in its global reach—and in the destruction of lives.