Request for clarification:

Do you mean the actual slave trade practiced by the United States after it had become independent from Britain? Or are you looking for the long history of this issue?

Estimates on the volume of the Atlantic Slave Trade vary, but most scholars accept tend to accept the estimates provided by David Eltis, et al, in the Trans-Atlantic Slave Trade Database. The estimate is that 12,500,000 slaves were embarked onto vessels along the African Coast, and 10,700,000 disembarked in the New World. This excludes those brought from Africa to Europe, and the huge number of slaves who died on the journey to the coast (Miller, Way of Death, pp. 140-153). Of all the slaves disembarked in the New World, approximately 500,000 arrived in the territory now called the United States. Of that number, only 106,000 arrived after the United States became independent (though that number will likely increase with the publication Craig Hollander's Ph.D. thesis on the illegal slave trade).

Finally, /u/EvanRWT is correct that you need to be specific about where in Africa you are talking about. Estimates from the Eltis database show a rough breakdown of the following:

Slave trade to the United States

Senegambia: 14,000

Sierra Leone: 24,000

Windward Coast: 6,500

Gold Coast: 20,000

West Central Africa: 32,000

Other: 10,000

Here is a map of the trading regions.