The federal health insurance marketplace has been getting heavy traffic since the Thanksgiving weekend, according to the administration, receiving a million visitors on Monday. That's twice as many visitors as the site had been seeing, officials said, resulting in higher error rates and longer wait times.

Still, the site did not crash and processed at least 18,000 new enrollments, according to the administration. About 13,000 users ended up in the queuing system designed to avoid overload. Those users will get an email asking them to return to the site at a less busy time.

The numbers show that Healthcare.gov's capacity has improved since its rocky launch. However, the site is still producing errors in the data sent to insurance companies that could be causing people to enroll in the wrong plans. That means some customers may have trouble next month when their health insurance is supposed to kick in.

Americans must purchase health insurance by December 23rd in order to be enrolled by next year. Most people must have health insurance in 2014 under the Affordable Care Act or face a fine.