A report published last week in the esteemed New England Journal of Medicine provided an overview of Obamacare’s first year, its successes and the challenges ahead. It also offered a yet another estimate of the number of people covered by the law: 20 million.

The NEJM report pulled a wealth of information, much of it already known by those closely following the law’s implementation but presented together by the journal, from think tanks and government agencies. It covered a range of topics, including the number of people covered, 2015 premiums, and the adequacy of provider networks for plans offered through the law.

But its bottom line was that millions of people have become insured under Obamacare.

“Taking all existing coverage expansions together, we estimate that 20 million Americans have gained coverage as of May 1 under the ACA,” the authors wrote. “We do not know yet exactly how many of these people were previously uninsured, but it seems certain that many were.”

They reached the 20 million total this way: 1 million adults under age 26 enrolled in their parents’ plan; 8 million enrolled in private coverage through the insurance marketplaces; 5 million enrolled in private coverage directly through their insurer; 6 million enrolled in Medicaid.

As the authors noted, some of these people were likely insured prior to the law. Experts have previously said they expect many of those enrolling directly with an insurer, for example, already had coverage. But the conclusion still stands, the NEJM authors said.

“With continuing enrollment … 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,” they wrote.

They also noted the disparity — and potential for even greater disparity — between the 28 states that expanded Medicaid under Obamacare, and the others that decided to not to following the 2012 Supreme Court decision.

“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,” they said, “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.”

They also observed some of the challenges still facing the law, particularly keeping premiums from becoming unaffordable and ensuring that customers are actually satisfied with the coverage that they get. It will likely take years to know the final outcome on those issues.

But the great challenge, and great unknown, is overall health care costs. The evidence there has been decidedly unclear, and it is likely simply too early to know what effect, if any, Obamacare is having.

“Developing and spreading innovative approaches to health care delivery that provide greater quality at lower cost is the next great challenge facing the nation,” the NEJM authors wrote.