So I've had a quick look, and in 1800 the number of slaves in the US was nearly 900,000. Which rose to 1,200,000 by 1810. Then 1,500,000 in 1820. To 2,000,000 in 1830.
Between 1800 and 1830 the number of slaves in the British Caribbean went from c.800,000 to c.600,000. So if staggered then...