Well, I haven't peeked at the results lately, but the dojos I've visited seem to be about 10-15% female, so you'd expect say one in ten yudansha to be female. Probably a bit less, since women are generally smaller and so may get slowed a bit due to injury, and certainly childbirth. Since many dojos don't have more than ten yudansha, I wouldn't be surprised many dojos don't have female yudansha. I would think the larger ones might hold true to the 10% or so.
What I find disturbing at my present dojo is the skill of female kyus seems less than that of their male counterparts at the same kyu level. I hope it is not to encourage an 'affirmative action' like promotion of women.
As a woman, I'd prefer senseis not focus too much on if we are making it high enough, any more than they'd be asking 'are there enough African American yudansha'. If you want more women to start with, there are ways to work on that, but don't worry if there aren't many blackbelts.