That's a really good question. I think the short answer is no. No one tracks it. Someone brings a book to market, and their job is to convince as many people as possible to buy it. Some of those people may or may not have a print disability.
I think to keep it in context, if you look at self-publishing today, Apple, Adobe and a multitude of other platforms provide the ability to self-publish. Provided that those platforms are accessible and that they design the platform to generate alternate format materials, the format that the consumer or I buy it in.... I either hook up my Braille printer to my computer and print it or listen to it on my Victor Reader or what have you. Then I bear the cost of making the content consumable in a format that I choose to digest, but the building has to be built to be usable and accessible, i.e., the building is the book.
Is that cost onerous? No. There's free software now. You can go to daisy.org, and you can download for free software that will take a Word document or an ASCII text file, or probably an XML or HTML file— XML is a protocol that publishers would be familiar with—and literally, at the click of a button, you can create a well-structured accessible book at no cost. Whether one person who's blind chooses to consume that or a thousand people choose to consume it, it doesn't matter. It's available in an accessible format.
We don't ask questions around how many people are going to be watching television with closed captioning. We don't ask that question anymore; we just make it available. One hundred percent of Canadian television has to include closed captioning.
Why would we ask the question that you just posed, sir? It's unfair and unnecessary. The fact is that all content should be made accessible, end of story.