In 1860, out of a Southern population of 9,000,000 people, 4,000,000 were slaves. These are figures for all States, not just Southern States. AGGR. NO. OF SLAVES Total...3,950,546
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So where would you go to find those slaves, to Rhode Island? You'd go to the states in the South where slavery was legal and encouraged.
You may want to download the Helper volume so kindly linked by x. It's a classic.