In the context of women being literal sex slaves, even 5 or 10% would constitute a high percentage that gave regular purchasers a reasonably high chance of encountering one. (I wasn't intending to play that semantics game, but it's worth noting regardless.)

I think it's pretty high. There are legal places that have some safeguards in place, but in most of the world there's just nowhere for a woman to say "I'd like to try sex work" and get an opportunity to do so. It's inherently the people on the fringes of society, the runaways with nowhere to go who end up reliant on a predator, the people addicted to drugs that have no way to get their next fix, the people trafficked. Even the "high class" stuff, because there isn't any legitimate entry point, is relying on tactics like calling it a modeling gig and then propositioning the models, and there's an inherent element of coercion* to that as well.

*Coercion isn't the perfect word choice but I'm blanking on a better one. Even if the intent isn't explicitly to manipulate the women, the result is a lot of wild emotional swings, then a pitch when you're still under their influence. And we've seen examples of people taken overseas and having their passports taken away, even by an NFL team. It definitely happens at a far more frequent level than we should be comfortable with.

I also know that male prostitution is a thing, and "women" isn't comprehensive. But it's mostly women.