Final answer:
The Southern United States had the greatest number of slaves back then due to its reliance on slave labor for cotton production.
Step-by-step explanation:
The region where you would most likely find the greatest number of slaves back then is the Southern United States. During the time of slavery, the Southern states, including Alabama, Arkansas, Florida, Georgia, Louisiana, Mississippi, South Carolina, and Texas, had a larger population of both slaveholders and enslaved people. The Southern states were the epicenter of cotton production, which relied heavily on slave labor.