I’d say that is the ‘wrong’ question. The important thing is not which disability is the most common, but which disability causes the most problem when a web page doesn’t take it into consideration.
The most common disability is probably visual impairment, if you include all of us who need corrective lenses (plus those with more severe sight problems, of course). But for most of us it’s not a major obstacle. With glasses or contacts we’ll get by fine. Even without them we can often manage by zooming the page, something contemporary browsers are all capable of.
People with very poor eyesight, but not completely blind, rely on screen magnifiers to use the web. Bad layout is a big problem for them, since they see only a small portion of the screen at a time (like looking through a keyhole). Similar layout failures cause problems for people with motor skill impairments, which may also include those with chronic and severe pain in joints or muscles, like rheumatism.
Colour-deficient vision can be a serious complication, but only on certain sites. It’s relatively common (~8% of the male population, ~1% of females). Poor colour contrast can cause problems for this user group, but also stupid instructions like ‘press the green button to confirm’.
Then we have the ‘difficult’ problems: content! Many users have some form of diminished reading capability. The reasons vary, from disabilities like dyslexia and ADD/ADHD to brain injuries and aphasia. People who are deaf since birth or early childhood usually cannot read as well as hearing people. Those who aren’t reading in their first language are also at a disadvantage. And people who simply don’t read much have, of course, more difficulties reading than us bookworms.
And these problems cannot be fixed simply by applying some clever HTML tags or attributes, or by doing the graphic design with some degree of competence. They require copywriting skills, often beyond those found in most web designers/developers/authors.
I realise that I’m not answering your question, and in fact I’m not going to. I think that if a job is worth doing, it’s worth doing well. That means I wouldn’t ‘adapt’ for one disability and ignore all others, because that is anathema to my entire being.
I just wanted to highlight that you shouldn’t be looking at sheer numbers, but at the consequences. If 20% of your visitors experience mild problems and are able to overcome them using built-in browser functions, it’s less of an issue than if 2% of the visitors are completely locked out.
And to those who are bound to chime in with comments about ROI and how it ‘just ain’t worth it’ to build accessible sites, I’ll just ask them to take their callous, greedy comments elsewhere. Remember: there are no pockets on the shroud.