Most people assume that when someone is described as "disabled" they are un-able: unable to hold a paying job, especially. The very word, "disabled" is actually repugnant to many people, as the unfortunate connotation is "inferior." This is a near-universal attitude, although many people with disabilities (PWDs) are actually holding jobs worldwide. Safety professionals who think they don't have any PWDs in the workforce (who will need some additional consideration) are almost certainly mistaken.
Additionally, there are worldwide movements to get more people with disabilities in the workforce. PWDs themselves are pushing for inclusion in all of society, including the workplace. Governments are finding that they have two reasons for disability inclusion: simple human rights issues, and economics. It costs more to pay people to stay home when they could work with some reasonable accommodations. In the unlikely event that you really don't have any PWDs in your workplace, you most likely will at some time in the near future.
How many PWDs are there? From my research, nobody really knows, in part because the definitions of disability vary greatly. For example, people in the U.S. can be declared "disabled" by the Social Security system and thus be entitled to disability benefits, while simultaneously eligible to file suit under the Americans with Disabilities Act against an employer or potential employer. This isn't as contradictory as it seems, as the Social Security system doesn't consider the possibility of reasonable accommodations in the workplace, and is more realistic about the discrimination that PWDs actually face when trying to get and keep jobs. No doubt other countries have different definitions of "disabled" in different branches and agencies of their governments.
To some extent, disability is in the eye of the individual and the beholder. People who actually do need disability accommodations are not requesting them because they don't think that their arthritis, for example, is actually "disabling." Different conditions are defined as "disabling" based on people's prejudices and understanding (or lack of it) of the actual impact of the condition. Conditions change, too, and a condition may be disabling at some times and not at others for the same individual.
There are also people who are temporarily disabled, and that number varies hourly. Conditions in the same individual may not be disabling in some situations, profoundly disabling in others and an actual safety asset in still others. So how many people with disabilities are there? It doesn't really matter. There are lots of us, and the number is growing rapidly because most countries have an aging population and we are saving the lives of an increasing number of trauma victims who would have died before. Most of these people will survive with residual disabilities.
