From BBC:
The exact numbers of Africans shipped overseas during the slave trade are hotly debated—estimates range between 10 and 28 million. What is undisputed is the degree of savage cruelty endured by men, women and children. Up to 20% of those chained in the holds of the slave ships died before they even reached their destination. Between 1450 and 1850 at least 12 million Africans were taken across the notorious Middle Passage of the Atlantic—mainly to colonies in North America, South America, and the West Indies. The Middle Passage was integral to a larger pattern of commerce developed by European countries. European traders would export manufactured goods to the west coast of Africa where they would be exchanged for slaves. The slaves were then sold for huge profits in the Americas.
More here. (Note: One daily post throughout the month of February will be devoted to commemorate African American History Month)