I don't have numbers, of course, nor even proportions, but I suspect that the number is small.
But more important is the question of how such a person would be hired and what sort of position they could hold. The problem is that a school or university wanting to employ such a person needs some assurance that the person is competent for the position. Not holding any degrees makes that more difficult in general.
But some people have excelled in a field with no formal education. That isn't especially unusual, actually, but not a lot of them will want to teach. But some will. Many will want to do it only part time and money isn't the driver for them, so a position as an adjunct can be attractive both to them and to a university. But no one will be looking at it as a career path.
I suspect that Harvard would be happy to have Bill Gates teach a course occasionally - probably not in BASIC language, though.
Luminaries in a field are probably offered such courses fairly frequently, I'd guess. But especially if the institution is looking to establish a long term relationship with future funding in mind.