Obamacare is now the 2.1 million person plan as the new Affordable Care Act health insurance plans selected by those people kick in for the first time on New Year's Day.
"Tomorrow is New Year's Day, and it's a new day for health care for millions of Americans," said US Health and Human Services Secretary Kathleen Sebelius on Tuesday as she announced that more than 2.1 million people had enrolled in private Obamacare insurance exchanges as of last Saturday.
But officials said they do not know what percentage of those people have actually paid their first premiums for the new plans, which is required to actually be enrolled. In many cases, those premiums are not due until Jan. 10.
Sebelius also noted that another 3.9 million people have been deemed eligible for the government-run Medicaid program via those federally and state-run Obamacare marketplaces by the end of November, the last reporting period for that data.
And she said HHS was committed to getting even more people signed up since "we're just halfway through a six-month open enrollment period."
"For many of the newly insured — people like Molly from Charlottesville, Va., Mark from Austin, TX — it will be the first time that they can enjoy the security that comes with health coverage," Sebelius said.
Sebelius announcement came a day after CNBC.com revealed the 2-million enrollment plateau had been topped. About half of the enrollments, or at least selection of plans without a premium being paid, came on HealthCare.gov, the federally run Obamacare insurance exchange that sells plans in 36 states.
In addition to clearly being pleased to report that millions of people have been enrolled — three months after the botched launch of HealthCare.gov — Sebelius underscored the fact that the plans those people selected, along with effectively all other insurance plans in 2014, will have new standards mandated by the ACA.
Those standards include no pricing discrimination against women or for people with pre-existing conditions, and mandated coverage without co-payments or deductible payments for things including preventative care.
"Starting tomorrow, being a woman will no longer be a 'pre-existing condition,' " said Sebelius.
Although the 2.1 million or more enrolled is well below the 3.3 million people federal officials last fall projected would be signed up by Dec. 31, it represents a strong rebound from the dramatically low level of enrollments seen in October and most of November, which reflected the serious technological problems on the federal HealthCare.gov exchange and several state exchanges.
Experts said that with three more full months of open enrollment in Obamacare insurance left to go, millions more will likely sign up, possibly enough to top the 7 million total sign-ups that officials had projected last fall before the HealthCare.gov launch debacle.
(Read more: "Direct" online enrollment in Obamacare stymied)
"I wouldn't be surprised if we exceed 7 million [nationally] by March," said Timothy Jost, a law professor who specializes in health-care legal issues at the Washington and Lee University School of Law. "I'm encouraged by the fact that we've got close to 2 million people signed up [officially] and in time we're going to get a lot more people signed up by March 31."
Jost noted that major national and state-based ad campaigns by supporters of the Affordable Care Act, which were supposed to launch in the fall, were suspended because the problems on HealthCare.gov made it unwise to encourage people to go to a website that was effectively not working.
With the resolution of much of the tech troubles on HealthCare.gov—which sells Obamacare insurance in 36 states—"there's going to be a very aggressive effort to get people to sign up" beginning in January, Jost said.
On Sunday, the head of the Centers for Medcare and Medicaid Services, which operates HealthCare,gov, announced that "more than 1.1 million people enrolled in a qualified health plan" on HealthCare.gov "from Oct. 1 to Dec. 24."
"More than 975,000 of those enrolling" did so in December, noted Marilyn Tavenner, administrator of the CMS. "December enrollment so far is over seven times that of October and November."
(Read more: The doctor is ... a felon, and still OK'd to receive Medicare money)
"Everyone said all along that people were going to wait until the last month to enroll," said Washington and Lee's Jost.
But he added that a "big problem is that we just don't have any idea how many people would have enrolled if people could have enrolled in the first two months."
"The numbers tell us that the website [HealthCare.gov] didn't work for the first two months," Jost said.
Jonathan Wu, chief analyst at the consumer finance website ValuePenguin.com, said the 2.1 million enrollment benchmark is "pretty good considering how many problems they had, considering they lost almost two months of enrollment."
"They should be pretty pleased," Wu said.
But now, Wu said, the "tricky part" is continuing what has become a relatively strong pace of enrollments into the first quarter of 2014.
"At this point," Wu said, "anybody who really needs insurance has probably gotten it."
(Read more: California's in a Golden State with Obamacare)
"Now you have to convince the rest of the people why this is a good deal for you," he said. "I think the heaviest list is people who didn't have insurance before, and didn't see the need for insurance."
Many such people will be younger, less-affluent adults, whose tight budgets and relative good health will make buying insurance less of a pressing need, Wu said.
"My concern is whether younger people will find this to be a really good deal or not," he said.
Most adult Americans who don't have health insurance in 2014 through either their employer, the Obamacare exchanges or government health programs like Medicare and Medicaid, are subject to a tax penalty of $95 or 1 percent of their income, whichever is higher.
Wu said it remains an open question for younger adults whether that penalty, known as the individual mandate, will be "enough to push them into the market" and buy insurance they didn't previously have.
—By CNBC's Dan Mangan and Jodi Gralnick. Follow them on Twitter @_DanMangan and @jodigralnick
