They're not at all prevalent, basically. They certainly existed and you sometimes see white supremacists bring up Anthony Johnson as the First American Slaveholder Ever as though this is a relevant point rather than purposefully misleading trivia and factually incorrect. But white supremacists and good history go together like vampires and sunlight. The very short version of that is we have known white enslavers prior to Johnson enslaving anybody -Samuel Maverick, up in Massachusetts, is the first we know of by name and a horrifying story presented as comedy about how he ordered two men he enslaved to rape the one woman he enslaved- and even if he were literally the first it doesn't change the fact that American slavery is staggeringly, overwhelmingly a system where whites enslave blacks and torture them for profit and pleasure, incidentally. I bring it up because it speaks to the prevalence. Another misleading thing you sometimes see is that free blacks, proportionately, are more likely to enslave people than whites. This may be true (though that's going to vary a lot place to place) but still amounts to looking at a very small number of individuals, literally a tiny fraction (black enslavers) of a small fraction (free blacks) of the population and ignoring the four million black Americans actually enslaved by whites. Such a maneuver is impossible to attribute to good faith.

But that does bring us to a broader, important point. Yes, black Americans enslaved other black Americans. Some of them, a decided minority, did so on a large scale and clearly for personal profit. The vast majority of these -I don't know of any prominent exceptions- were black by the racial mores of the era but were actually the sons of the white enslavers who raped their mothers. These individuals were set up by their white relations and allowed some access to white spaces, achieving a degree of liminality, becoming "brown" as opposed to "black" when there were few enough of them that their numbers don't threaten the general system of racial slavery in the minds of whites but there are also enough of them around to produce something like a social circle or small community amongst themselves. This is most common in and around Charleston, South Carolina and New Orleans. That's the Lower South, where free blacks are vanishingly rare. In the Upper South, such things are not generally allowed because free blacks are more numerous and their visible success thus more threatening to whites who in response insist that all black Americans be treated essentially as slaves and some have an anomalous, deeply troubling state as slaves without masters. There are always individual exceptions, but that's the general pattern.

So there are those enslavers, who orient themselves toward and to some extent are tolerated as part of white society. Some of them own slave labor camps and do quite well for themselves. Others end up working as mechanics (we would say artisans or skills tradespeople) or of even more modest means and enslave a far smaller number of people. These people may or may not be related to a nearby white enslaver. What they definitely are is far more common than black enslavers who grow cotton off the bloody business ends of their whips, fertilized by screams.

Many instead came to their freedom by self-purchase or successfully stealing themselves and establishing themselves in a city where they blend in with the small number of free blacks who live there, many of whom have similar origins. They could enslave just a few people. Sometimes that means they're still exploiting those people just like the others, but it can also mean that they've bought their children, spouses, parents, siblings, or other loved ones and carried them into de facto freedom. It's expensive and difficult to free a black American legally, often requiring a special act of the state legislature and generally beyond the means of a black enslaver.

Buying that person, which makes them legally one's property, and then treating them like a family member instead of chattel property is an option that many definitely took. We know both from looking at the demographics of the enslaved (lots of adults with people the right age to be their children or spouses, for example) and from the efforts they made to protect their loved ones from being re-enslaved upon their deaths. Are you an enslaver if you buy your child to spare them torture and sexual violence? Legally, yes. But if you don't treat them as enslaved in an environment where freeing them is not a realistic option, then most of the substance of slavery is not there. People in this situation are generally doing the best they could, which is awful because slavery is unimaginably horrific and that's exactly how white Americans liked the system to be.

All of which goes back to how misleading the stat that more black Americans, proportionately, owned people than whites is. It's a transparent attempt to throw up one's hands and distract from the actual history of slavery in the United States by blaming it on its actual victims. i don't know how one could genuinely confuse a relative trying to protect a loved one from a regime of violence, torture, and rape and someone enthusiastically subjecting others to that same regime for profit except by engaging in bad faith and/or coming in with the conviction that free blacks are dangerous and must be controlled, and that slavery is a morally permissible way to do that. In other words, one must believe just as the white enslavers did, that black people are simply inferior to whites and rightly subject to their arbitrary control through violence for the profit and enjoyment of said whites. Those ideas have long outlasted American slavery itself.

Sources

The definitive work on black enslavers in the US is Koger's Black Slaveowners: Free Black Slave Masters in South Carolina, 1790-1860. Anthony Johnson's story is discussed in Berlin's Many Thousands Gone, which also explains the nuances in how free black people were treated in the South. His Slaves Without Masters is the standard text on that specifically, from which I took the turn of phrase. For actual first enslavers in the US, you can find an account of the first we're sure we know the name of in passing in Warren's New England Bound.