I'll just try to piece together what little data we have to estimate something.
In a poll on attitudes to interracial dating done back in 2005, they asked people about their dating history. 24% of white women admitted to having dated a black man in there.
https://news.gallup.com/poll/19033/Most-Americans-Approve-Interracial-Dating.aspx
A similar study of students in California from 2000 found that 28% had dated a black guy.
https://www.researchgate.net/publication/12059869_Interracial_Dating_Attitudes_and_Experience_Among_American_College_Students_in_California
I'd take the national poll as being a better measure of things as it had more people, and a wider variety of women.
But that's just dating, what about adding in casual sex? It's definitely more likely that a white woman would fuck a black guy than date one. So there is this other study about black and white women's reaction to watching porn. The main point of the study is not the focus here, but the part where they gathered information on the women participant's dating and sexual history are. In it, the white women admitted in group 1 that 33.3% had dated interracial while 80.8% had had interracial sex. In group 2, 24.3% had dated interracially and 77.8 had had interracial sex. Of course this didn't just mean black men. But it gives an idea of how much more likely interracial sex is compared to dating.
https://sci-hub.yncjkj.com/10.1300/j514v19n02_02
If we're using the dat from the two groups we got, it averages out that for interracial sex was 2.8x more common than interracial dating. So if we apply that to the poll results of 24% of white women having dated a black man, that would mean that 67% have dated or slept with one. Of course that is also almost 20 years ago. Interracial relations are much more accepted and common now than before. If we play it safe and apply just a modest increase to the old dating data, and put it to 30% instead of 24 , then perhaps you could say that around 85% of women in the current year have possibly slept with a black guy.
That's the best I can guess.