Some 12 million people could gain health coverage this year because of the law, if congressional auditors' predictions don't prove overly optimistic.
Even so, tens of millions still would go without.
That's partly because of immigrants in the country illegally; they aren't eligible for marketplace policies.
Some of the uninsured will not find out about the program in time, will find it confusing or too costly, or will just procrastinate too long. Some feel confident of their health and prefer to risk going uninsured instead of paying premiums. Others are philosophically opposed to participating.
Figuring out just how many of the uninsured got coverage this year won't be easy because the numbers are fuzzy.
The administration's enrollment count includes people who already were insured and used the exchanges to find a better deal, or switched from private insurance to Medicaid, or already qualified for Medicaid before the changes.
Some who sign up will end up uninsured anyway, if they fail to pay their premiums.
The budget experts predict enrollment will grow in future years and by 2017 some 92 percent of legal residents too young for Medicare will have insurance.
But even then, about 30 million people in the United States would go uncovered.
Some are left out
A gap in the law means some low-income workers can't get help.
The insurance marketplaces weren't designed to serve people whose low incomes qualify them for expanded Medicaid instead. But some states have declined to expand their Medicaid programs. That means that in those states, many poor people will get left out.
People who fall into the gap won't be penalized for failing to get covered.
Some others are exempt from the insurance mandate, too: American Indians, those with religious objections, prisoners, immigrants in the country illegally, and people considered too poor to buy coverage even with financial assistance.