I wasn't referring to prison labor although that is a valid discussion too. It's ironic that the "land of the free" only leads in incarcerated citizens, obesity, and diabetes per capita.
I also don't have a reference on hand. I listened to an NPR radio segment that blew my mind. There's a bunch of information from reputable sources but finding a specific number can be tricky. First of all, it's estimated because it's obviously unknown. Also, defining the "bounds" of slavery inherently changes the count. There is a fairly recent citation that states there are 400,000 enslaved people in the United States on any given day.