The Wall Street Journal's Numbers Guy tackles the question:
The Census Bureau estimates that the number of uninsured amounts to 45.7 million people. But the agency might be over-counting by millions due to faulty assumptions...
Even though legislation won't cover many of them, illegal immigrants are especially difficult to enumerate: Few raise their hands to be counted. Prof. [Jonathan] Gruber estimates they make up about 13% of the uninsured today, or nearly six million people of that 45 million number...
Of the rest, some people are eligible for health insurance but don't know it and many can afford it but don't want it. About 43% of uninsured nonelderly adults have incomes greater than 2.5 times the poverty level, according to a report released Tuesday by the business-backed Employment Policies Institute.
He left out a few things, though.
The estimate of 46 million uninsured, which comes from a less-than-ideal government survey, has been the occasion of a fraud on the public. For 20 years, the Church of Universal Coverage told us that 40-some million Americans are uninsured for the entire year. Then, experts including the non-partisan Congressional Budget Office said that no, 40-some million is the number who are uninsured on any given day, and a lot of those people quickly regain coverage. The number of Americans who are uninsured for the entire year is actually 20-30 million. Yet the Church of Universal Coverage kept using that 40-some million estimate as if nothing had happened – even though the meaning of that estimate had completely changed.
The Congressional Budget Office also reports that as many as 15 percent of those 20-30 million chronically "uninsured" are eligible for government programs, so they're effectively insured.
According to economists Mark Pauly of the University of Pennsylvania and Kate Bundorf of Stanford, as many as three-quarters of the uninsured could afford coverage but choose not to purchase it. Again, according to the Congressional Budget Office, 60 percent of the uninsured are under age 35, and 86 percent are in good-to-excellent health.
Government intervention has made health insurance unnecessarily expensive for them, so these folks quite sensibly don't want to be ripped off. Mandating that they buy coverage is really about hunting them down and taxing them.