Greg Mankiw asks and then answers, that he is not sure, but it certainly is less than the 47 million number cited by Hillary Clinton.
Back in the early 1990's, the number that was often cited by national health care proponents like, oh.... Hillary Clinton, was 45 million uninsured Americans. But as Mankiw points out, a fair number, of those people CHOOSE to be uninsured. If you are making a conscious choice, then while you may be uninsured by definition of not having any insurance, you are not really uninsured because you have no ACCESS to health insurance.
That is a far cry from being uninsured.