It's very, very difficult to say for certain, but it's likely several hundred thousand people were traded around the Americas as slaves before the abolition of the slave trade - though it was probably much, much rarer for mainland American slaves to be taken to the Caribbean than vice-versa.

The records we have for the inter-American slave trade are much more fragmented and incomplete than those we have for the transatlantic trade as a whole, in no small part because the trade itself was a much more fragmented thing. Whereas vessels crossing the Atlantic would be carrying hundreds of people out of West Africa to the Americas, the ships of the inter-American trade were rarely of comparable size; it would be much more common to move dozens rather than hundreds of people at a time. Tracing these journeys is complicated by the fact that the men, women and children moved through the inter-American trade were either only ever fleetingly in the first colony they landed in as a holding port (and so not recorded as enslaved people arriving in that colony), or were sold on to intermediate traders at the colony whose movements did not produce the same kind of immense record trail that the vast transatlantic trade did. In addition, enslaved people in the inter-American trade were very frequently carried on general trading ships rather than specialist vessels in perhaps the most literal example of how Atlantic slavers really did regard the victims of their trade as property.

George O'Malley collated together a very impressive collection of almost 7,700 known inter-American slave shipments. O'Malley has very dilligently checked his work against the much larger collection of nearly 100,000 estimated trips slave ships made from West Africa to the Americas through the history of the transatlantic trade, but he admits that the comparative scarcity of records and the diversity of activity in the inter-American trade makes arriving at a definitive count of victims very difficult indeed. But generally speaking, the pattern O'Malley discovered - which reaffirmed what we already largely thought to be the case - was one in which the British Caribbean, especially the islands of Jamaica and Barbados, was the nexus for an intercolonial trade that ferried people throughout the rest of the New World. Barbados in particularly played a very considerable role in the early colonial period in providing slave labour to ports in Virginia, Maryland, New York and later the Carolinas. Barbados was one of the earliest and most enthusiastic adopters of slave labour in the British Caribbean and most probably also played a role in providing forced labour to support colonisation efforts in other islands as well during the early period, before it was practical for such fledgling colonies to independently import slave labour from Africa on any meaningful scale.

The nature of Britain's colonies in North America meant that there wasn't very much incentive to traffick people from the mainland to the Caribbean compared to vice-versa. The majority of the British Caribbean isn't as close to mainland America as many people imagine it is. It's about 1,800 miles of ocean between Barbados and the Virginia coastline, which is still about half the distance between Barbados and the modern-day Liberian coast. Although certainly on the whole much safer and much more straight forward, sailing from Barbados to Virginia and back is still a considerable journey. The kind of goods being manufactured in each part of the Americas also created a very sensible flow of trade. You might have heard the term 'triangular trade' in reference to slavery, and the inter-colonial trade formed part of it. Vessels would journey from West Africa to the Caribbean to sell enslaved people as forced labour for the plantation economy, and use the profits to purchase unrefined sugar to take to the mainland, where they would sell it for use in making refined sugar products like rum. They would use these profits in turn to purchase rum, timber, clothing and so on that could be taken back to Europe, where these goods would be sold on and used to purchase other goods and equipment for the ship. These goods could then be taken to West Africa to trade in exchange enslaved people, who are then taken to the Caribbean where the cycle starts over again. Whilst the ships of the inter-American trade were generally poorly suited to full engagement in the triangle trade, they were nonetheless part of it at a global level, able to complete the Caribbean - American leg of the journey and return goods to the Caribbean for shipment to Europe. In addition, the early slave trade in English colonies was more tightly regulated from a commercial perspective. The Royal African Company - which takes it name from the fact it was established by the man who would become King James II - had a legal guarantee to a monopoly on slave trading that gradually diminished in effectiveness over the course of the 17th century, and was repealed in full by the start of the 18th. The RAC monopoly in the early colonial period allowed them to heavily dictate the direction of travel the slave trade took.

We also know that some slave traders and slave owners had a preference for forced labour from the Caribbean over West Africa. There was a perception among some on the mainland that slave labour with experience of living and working on the Caribbean islands would have a much easier time adapting to conditions in the Americas, already being accustomed both to climactic and social conditions in the region, compared to enslaved men and women taken directly from West Africa. But the British Caribbean was an extremely labour-hungry region itself capable only of providing so much surplus labour to mainland colonies, and could never have possibly fulfilled the demand of growing settlements in mainland North America. Additionally, prospective traders in the British Caribbean had other destinations that were much closer to home and much more suited to a trade that depended primarily on the efforts of more modestly-sized ships. Even though much of the trade had to be conducted illicitly colonies in South America and the Spanish Caribbean were a major destination for inter-colonial slaver traders from the British Caribbean whenever political conditions would allow for reasonably safe travel, whilst French colonies in the 17th century had such difficulty sourcing forced labour that many English traders found the risk of smuggling slave labour into those colonies much more profitable than the intermediate trade around England's possessions. By the heyday of British involvement in the slave trade and into its final years, Britain had modified its commercial laws and regulations to implicitly make this kind of inter-colonial trading in the Caribbean easier, reflecting how hugely profitable it could be.

Perhaps the one exception to this pattern comes in the early colonial period, when the English transatlantic trade was still in its relative infancy. In the early decades of North American colonisation England was not yet dependent entirely on slave labour from Africa for pursuing its imperial ambitions. Instead, the very early colonial period is marked by a hybrid of forced labour systems that included the exploitation of voluntary or semi-voluntary indentured servants from England; indentured political and criminal prisoners; enslaved Africans; and enslaved Native Americans. We know that many tens of thousands of Native American people who had been forced into bondage by colonists in the future United States were sold onto slavery in the British Caribbean in the 17th century in particular, sometimes as part of an exchange for African slave labour. This practice was driven particularly by the explosion of sugar cultivation in the British Caribbean - the same cultivation that would later help create favourable conditions for a much more sustained trade of people flowing into mainland North America. But this was an exception rather than a persistent trend throughout slavery's history. Generally speaking, people were taken from the British Caribbean to other parts of the Caribbean or to the colonies on the mainland, and it was much less common for the trade to flow in the other direction.

O'Malley's Final Passages: The Intercolonial Slave Trade of British America, 1619 - 1807 is probably the best book to look up if you're interested in learning more about this. Cecile Vidal has also very recently published a book on the history of New Orleans called Caribbean New Orleans: Empire, Race, and the Making of a Slave Society, which is a very good exploration of how colonies in the future US can and should be understood for at least their early history as very much representing an outpost or extension of what was happening in the Caribbean, rather than a completely different colonial enterprise removed from Caribbean affairs. Quite a lot of is currently being written about the history of early colonial development in North America, especially around the Carolinas and Virginia, and there are no shortage of titles and debates to get into that I can direct you towards if you have any particular interests there as well.