The forum doesn't remember who looked at a thread. It just counts the number of times someone loaded it. Go hit f5 a bunch of times and I'm pretty sure the count goes up, though I haven't tested.
With the caveat that it's just a well-informed guess, you get to 20000 people in the U.S. like this. First, start here from some blindness statistics, through which we know that 1M people are blind, that is, definitely without enough vision to go play sighted games, which make audiogames look like crap. Before we do anything else, this already limits the market. The rest of this is more broad estimates not facts.
Get rid of all the people who went blind later. They are going to have much bigger problems than audiogames: it will take them many years to reach proficiency with a screen reader, they didn't grow up with games, and they probably have bigger problems. "Use all your skills you don't have to play audiogames" is a big ask, they're probably not going to bother.
So that limits us to children. We can get info on that here which says 63k currently, 4k braille readers, and 6k audio readers. So that gives us an estimate somewhere between 63k and 10k as to how many people at any given time are blind enough for this sort of stuff; let's split the difference and call it roughly 30k students in school. Some of the other numbers on that page are interesting, but the APH statistics are a good proxy, because it counts the number of people capable enough to be doing school at all, which are probably the number of people who are curious and interested enough to go seek out communities like this, and who have the skill to do so.
Audiogames and the tech for accessible games, screen readers, etc. have all been around for 20 years, and we can probably safely assume that the number of blind people is stable over time. This means that one generation has had the chance to get all the way through school and become adults. So let's double our 30k number to account for that to 60k, then say that maybe we've got 40k or 50k adults older than that, who somehow have the skills and interest or whatever. This puts us at, say, 100k.
Now, we go to "who plays games in sighted land" stats. A quick Google gives us this which says it's about 70%. This immediately knocks us down to 70k.
Next, we know that most blind people make minimum wage, and that the blindness unemployment rate is anywhere from 30% to 70%. What we're interested in is "how many people have money?" So again, let's split the difference and say 50%--except that frankly most blind people are going to be working minimum wage, so they won't have enough money left over to go buy a bunch of luxuries. SO let's start by splitting in half again to say that the total number of blind people who would buy games is 35k.
After this, why I think it lower is more subjective. First, each of those 35k people is probably only going to buy 1 or 2 games a year even if there was a glut of content. You might be able to run something like my hypothetical MMO project at cost because reasons, and otherwise scrape a bit more out using subscriptions, but broadly speaking before you even factor anything else in there isn't money to be had. A lot of those are children, whose parents are probably just going to go "wtf, why would I buy you a PC and some games when there's nothing to play, how about a puppy instead" or whatever.
To justify why I don't think this number is as high as 35k with a more mathematical argument, let's start by saying that they'll each buy 1 game a year for $15 without fail, provided there's a game to buy. This puts the total value of the market at around $500000 before taxes; call it $400000 after taxes and company overhead (yes, it is that much; if you're employed there's actually a 15% tax even before income taxes, most of which is taken on by your employer). If the number were at 35k, that would fund a few audiogame devs full time. But this doesn't happen. If the money were lying around to be picked up that easily, someone would have done so and we'd have Manamon 10 or Shades of Doom 5 just because that's enough to fund it. But it's even weirder than that--if this estimate were right for the U.S. then we should have a multimillion dollar industry, so in order for it to be right, where are the big audiogame production studios? We should at least have one. we don't, which is some pretty big evidence that these numbers are way higher than it actually is.
But next--say there's 30 accessible sighted games. Say that all of them are equally interesting and that we all go buy one without fail and somehow the 35k estimate is right. You just divided your market by 30. You don't even get one extra dev at cost per studio assuming the 35k number, but the 35k number can't be right. Now maybe the 35k number might be right with some big promotion and social media drives that grow the audiogames market, but everyone is already aware of audiogames, so I doubt that can work--and, again, that costs money. So as soon as you do enough games, that's it. We can't be enticed by better graphics, which means that if we have an accessible game we can all play it for 20 years, which means that the more games get released that are quality, the smaller the market gets. Permanently. This is literally the worst possible market situation you can possibly be in. It's like saying you want to sell cookies but 95% of the country has diabetes and also you're selling magic infinite cookie jars so that no one ever needs to buy your cookies again. And that's the end of that.
This is the market research for the mmo project I want to do, which I estimate might be able to bring in $20k annually, which is only enough to run at cost and get new content by using the fact that you can find art and writers for cheap in blindness land. The reason we don't have quality audiogames? No one to sell them to. The reason we don't have lots of accessible sighted games? No one to sell them to. And that's the end of that, really. We don't matter. We probably can't matter outside big studios doing altruism. At most, we can matter for 5 or 10 years, but only in the best possible universe, then people run out of things to sell us because the market forces that keep sighted people coming back and upgrading their consoles or whatever don't exist here. Programmers are expensive. Even if the audiogame market is somehow 20 million dollars, that's what just one small SF tech startup brings in every year.