The number of Obamacare customers keeps growing—which means the number of potential ex-Obamacare customers this year keeps increasing as well.
Nearly 11.7 million people nationally signed up for Obamacare health insurance plans sold through government marketplaces as of Feb. 22, U.S. Health and Human Services Secretary Sylvia Burwell said Monday.
Read MoreObamacare costs set to fall: CBO
That's about 300,000 people more than the 11.4 million sign-ups for 2015 plans reported just after the close of open enrollment on Feb. 15. The higher tally reflects effective extensions of the sign-up deadlines in most states that allowed people in the process of enrolling in plans at the time of the cutoff to finish.
The total number of people signed up for Obamacare plans is set to increase even more in coming weeks as most states allow people a special enrollment period for people who had been unaware until tax season that they face a financial penalty for having failed to obtain health insurance last year.
The current tally is about 5 million more than the 6.7 million people who were enrolled in Obamacare plans right before the start of open enrollment on Nov. 15.
"These numbers send a powerful message: the Affordable Care Act is working," Burwell said Monday at a White House event, where Obamacare supporters were thanked for their efforts during the open enrollment season.
"These numbers represent real people whose lives have changed for the better," she said. "While we know that number [of customers] will change as the year continues, we are pleased with the results to date."
Enrollments aren't considered official until people pay for their first month's premiums. And officials expect some people to leave their individual health plans during the year even as some others enter the market, as was the case last year.
Read MoreIRS rescue for Obamacare? Maybe
During the celebration of enrollment efforts and their results, Burwell brought out a long "selfie stick" to take a photo with herself and the crowd in the auditorium where she was speaking. But while that stunt tickled the crowd's funny bone, Burwell also underscored the risk that millions of those customers face from a pending Supreme Court case that challenges a key feature of the Affordable Care Act.
Plaintiffs in that case, known as King v. Burwell, claim that the subsidies given to about 7.7 million customers of the federal Obamacare exchange HealthCare.gov are not legal because the ACA only authorizes those tax credits to customers of an exchange operated by an individual state. HealthCare.gov serves 37 states currently.
Burwell noted that nearly 90 percent of HealthCare.gov customers get tax credits worth an average of $263 per month, and more than half of those subsidy recipients paid less than $100 per month for their health plans.
Almost 1.5 million Florida residents, more than 1 million Texas residents and more than 500,000 North Carolina residents alone get Obamacare subsidies to help pay for their monthly insurance premiums for HealthCare.gov-sold plans, she said.
Read MoreThe Obamacare challenge that won't go away
"Those who support this lawsuit believe that the law should be dismantled or repealed and they are content to roll back the progress that we have achieved together," Burwell said.
But she also said, "We are confident that we will prevail in the court case argued last week before the Supreme Court."
"The law is clear. The text and structure of the Affordable Care Act demonstrate that individuals in every state are eligible for tax credits," Burwell said.
But plaintiffs in the case and other Obamacare opponents say that the ACA, as written, only allows tax credits for customers of state-run exchanges, because the law only explicitly mentions those subsidies being given to such customers. The law does not explicitly say anything about those subsidies being awarded to customers of a federal exchange.
HealthCare.gov was set up to serve people in states that opted not to create their own Obamacare marketplaces. The Obama administration and its supporters say the contemplation of such a scenario by the ACA gives the administration the ability to issue tax credits for HealthCare.gov's customers.
Without those subsidies, few insurers would be inclined to list their plans for sale on the federal exchange, and few customers would be likely to buy those plans, Obamacare defenders argue.
The Supreme Court is expected to rule on the case in late June. Insurance experts agree that if the subsidies are taken away, millions of people will drop their HealthCare.gov plans because they won't be affordable at retail prices, and the remaining customers will face sharply higher premiums next year as insurers adjust for the exodus of many young, health enrollees.