HealthCare.gov website seen on Nov. 12, 2014. Adam Jeffery | CNBC

The Obama administration has reached a big Obamacare sign-up milestone, with less than three weeks remaining in the open-enrollment season. Officials on Tuesday announced that 9.5 million people have signed up for health plans sold through government-run Obamacare exchanges. Read MoreMedicaid expansion OK'ed for Indiana

The administration had said it expected that 9.1 million people will be paying for Obamacare coverage by the end of 2015, although it also had said that it expected as many as 9.9 million to have been signed up and paid for plans by the Feb. 15 open-enrollment deadline. The difference in the numbers stems from the fact that some people will end up not paying for coverage, or obtain coverage elsewhere later in the year. However, the Congressional Budget Office is projecting that 12 million people will be enrolled in Obamacare plans by the end of this year. For enrollment to be considered official, a customer must pay their premium for the first month. Enrollment numbers released Tuesday do not reflect paying customers.



The vast majority of sign-ups announced Tuesday came in the 37 states served by the federally run exchange, HealthCare.gov, which so far had enrolled more than 7.1 million customers in plans as of Jan. 16, according to data released Tuesday. The balance, or about 2.4 million people, signed up for individual insurance plans sold on the 14 exchanges being operated by individual states and the District of Columbia as of Jan. 17, according to the U.S. Health and Human Services Department. Both sets of enrollment tallies reflect customers who were automatically re-enrolled in the plans they had in 2014, and for customers who have selected new plans for this year. Read MoreObamacare is a huge help for mental health care

The biggest number of sign-ups came in Florida, whose residents have to use HealthCare.gov, and which has had almost 1.3 million people select plans. California, which is running its own exchange has 1.2 million sign-ups so far for 2015, according to officials. In a breakdown of the data, officials said that 87 percent of HealthCare.gov customers qualified for federal financial aid to help pay their monthly premiums, and that 35 percent of customers, or 2.5 million people, were under age 35. Young adults are sought after by Obamacare advocates because they help offset the costs of insuring older, less healthy customers. About 4.16 million HealthCare.gov customers were re-enrolled in a plan sold on that exchange, while 3 million were selecting a plan for the first time, officials said. Just before open enrollment began Nov. 15, there were about 6.7 million paying customers of Obamacare plans nationally.