In November 2014, less than a week before the marketplaces reopened for the 2015 enrollment period, Ms. Burwell offered a surprisingly modest estimate of the number of people who would sign up for insurance. She set a goal of having 9.1 million people on the rolls, with premiums paid, at the end of December 2015. Administration officials said Tuesday that they were on track to meet or surpass that goal.

Administration officials said they were confident that they would prevail in the pending Supreme Court case, King v. Burwell, and Ms. Burwell says she has not made contingency plans to deal with any disruption of subsidy payments. But the officials appeared to be eager to send a message — to the public and the court — about the hardship that could result from a ruling against the administration.

In its report Tuesday, the administration listed 11 states where more than 90 percent of consumers insured through the marketplace were receiving subsidies. Mississippi had the highest share, at 94.5 percent, followed in order by Florida, North Carolina, Wyoming, Louisiana, Arkansas, Georgia, Alabama, Wisconsin, Alaska and South Carolina.

Without subsidies, many people would be unable to afford insurance.

Of the 10.2 million people who were enrolled at the end of March, the administration said, 68 percent had selected midlevel “silver plans,” and 21 percent were in bronze plans.

While many people lost marketplace coverage because they failed to pay their share of premiums, some could have fallen off the rolls for other reasons — if, for example, they obtained insurance through their employers or became eligible for Medicare or Medicaid.

The government also ended coverage for some people because they could not establish their citizenship or immigration status to the satisfaction of federal officials.

On March 31, the government terminated coverage through federal marketplaces for 117,000 consumers who could not document their citizenship or immigration status. For similar reasons, the government ended coverage for 109,000 people in 2014.

In addition, the government reduced, increased or eliminated subsidies for people whose income was different from what they had initially projected. Because of these “income inconsistencies,” the government said, it altered the subsidy level for 223,000 households this year and 97,000 households last year.