There are more people enslaved today than at any time in human history. Including during the transatlantic slave trade days.

I would suggest that the definition of modern slavery now would have encompassed more than just the transatlantic slave trade in the past, such that the numbers would be much closer if not worse then than now. The definition of modern slavery encompasses practices which were very much happening back then too but which would not have been included in the then definitions of slavery. It's similar to how the definition of sexual assault encompasses a lot more now than it used to in the past. As our society has moved forward we have expanded our definitions to suit our evolving morality.