A recent succinct commentary describes the ~20 million people who gained coverage as a result of the Affordable Care Act/Obamacare; the exact number who were uninsured prior is not known.  While the article provides a clearer picture of this expansion, it also makes the point that there are many issues that need to be addressed including cost containment.  Other subjects:

• Cancelled policies
• Risk pools and premiums (lower than projected thus far)
• Narrow Provider Networks
• Crucial Younger Age Group Enrollment
• Small Business and Mandate
• Individual Marketplaces
• State Marketplaces
• Medicaid Expansion (limited in many states)

Here is the link –the entire report is worth a read: ACA -20 million Americans

Here is an excerpt:

Taking all existing coverage expansions together, we estimate that 20 million Americans have gained coverage as of May 1 under the ACA (Figure 3 Categories of Expanded Health Insurance Coverage under the Affordable Care Act (ACA).). We do not know yet exactly how many of these people were previously uninsured, but it seems certain that many were. Recent national surveys seem to confirm this presumption. The CBO projects that the law will decrease the number of uninsured people by 12 million this year and by 26 million by 2017. Early polling data from Gallup, RAND, and the Urban Institute indicate that the number of uninsured people may have already declined by 5 million to 9 million and that the proportion of U.S. adults lacking insurance has fallen from 18% in the third quarter of 2013 to 13.4% in May 2014.

However, these surveys may underestimate total gains, since some were fielded before the late March enrollment surge and do not include children. With continuing enrollment through individual marketplaces, Medicaid, and SHOP, the numbers of Americans gaining insurance for the first time — or insurance that is better in quality or more affordable than their previous policy — will total in the many tens of millions.

As we look to the future of the coverage provisions of the ACA and their effect on the U.S. health care system, several observations seem justified. First, as the number of individuals benefiting from the law grows, its wholesale repeal will grow less likely, although the law could still be importantly modified in the future.

Second, experience with the ACA will vary enormously among states. Those deciding not to expand Medicaid will benefit far less from the law, and since many of these states have high rates of uninsured residents and lower health status, the ACA may have the paradoxical effect of increasing disparities across regions, even as it reduces disparities between previously insured and uninsured Americans as a whole.17

Third, the sustainability of the coverage expansions will depend to a great extent on the ability to control the overall costs of care in the United States. Otherwise, premiums will become increasingly unaffordable for consumers, employers, and the federal government. Insurers who seek to control those costs through increasingly narrow provider networks across all U.S. insurance markets may ultimately leave Americans less satisfied with their health care. Developing and spreading innovative approaches to health care delivery that provide greater quality at lower cost is the next great challenge facing the nation.