The best estimates are that about 11 million people were shipped to the American colonies against their will and that about 10 million of them were Africans of some sort. Of these, about 6-7% ended up in the future United States. These estimates vary, with some going as high as twenty million, but the percentages are roughly consistent.

Brazil was by far the biggest destination for slaves, with the Portuguese colonies making up about 39% of the trade. All British colonies put together were a bit less than 29% of the trade, with the vast majority going further south than the future US. The Spanish were about 18%. The French about 14%. The remainder were the Dutch, the Danes, and a few other short lived or minor colonies.

Slaves were virtually always imported to colonies experiencing labor shortages. So, for example, there wasn't a lot of importation into Mexico because of its large native population. They were imported to some parts of Central America, but often only to specific districts that lacked a local population.

Perhaps more interestingly, despite making up 6% of the slave trade, African Americans make up about 55% of the African diaspora in the Americas. This is despite the fact that the United States was the second nation to ban the slave trade (Britain beat the US by a few months). The reality is that two factors led to this demographic shift. Firstly, America was less likely to allow mixing of races until very recently.

But the much, much larger factor is that American slavery had a significantly higher survival rate than almost anywhere else. American slaves had a life expectancy that was 60-90% that of a free white person (varying over time). In the Caribbean, the number was more like 18% and that was typical of elsewhere. American slaves thus lived significantly longer than their counterparts in other colonies and countries. They were also wealthier (relative to other slaves, not free Americans or even Mexican peasants) and in general had better conditions to form families. The African American population thus became self-sustaining relatively early on and has consistently grown. In contrast, the African population of (say) Barbados was basically static until the abolition of slavery. New generations were imported, worked to death, and replaced.

So that was what happened. They were mostly worked to death, with the survivors forming the core of the Afro-Latino, Afro-Islander communities. But while the African-American community grew from less than half a million individuals to more than forty-five million (an almost hundred times increase), those other communities grew from ten million to less than forty million (a less than four times increase).

American slave owners were aware that their slaves were better off, by the way. Right up to the end they were claiming they treated slaves better than foreign slave owners. This was probably true. But it was still, you know, slavery. It is entirely possible to admit a kidnapper is better than a murderer without condoning either. They also claimed they treated slaves better than Northerners treated factory workers, which is an interesting ancestry to certain politics in the Progressive Era. (And false, by the by. Millions of Africans did not voluntarily immigrate to the South to work on plantations.)

And as one last note, slaves eagerly adopted this quasi-patriotism about the superiority of American slavery... and then turned around and used it to make demands on the slave owners. After all, agreeing with Master about how well he is treating you is a great segue to talk about the quality of the food, clothes, and housing he's giving you. When you read about the shock slave owners had at African American hostility to the institution, it was often because their slaves had (understandably) chosen to lie to them. To paraphrase Hegel, no slave owner ever understands slavery but every slave does.

From The Transatlantic Slave Trade, The Atlantic Slave Trade: Effects on Economies, Africana: The Encyclopedia of the African and African American Experience, and The Slave Trade: The Formal Demographics of a Global System.