"We will not have those numbers until mid-November," Tavenner said, noting that the initial number is expected to be small.
Camp expressed concern about preventing a spike in premiums if not enough young adults enroll in coverage. Tavenner said health-plan rates for 2014 are "very competitive."
The successful enrollment of younger, healthier adults is considered vital to the success of the Affordable Care Act -- sometimes called Obamacare -- because their premiums are expected to help offset the cost of care required by older, sicker Americans.
During the question-and-answer period, Democrats on the panel urged their colleagues -- Democrats and Republicans -- to be patient and work together.
"We can't get caught up in the glitches and technical difficulties and lose our perspective," said Rep. Jim McDermott (D-Wash.).
But Republicans pressed Tavenner for assurances that the website would be ready in plenty of time for people to enroll for coverage, especially those whose health insurance policies have been cancelled because their health plans don't meet new criteria under the Affordable Care Act.
Dec. 15 is the deadline for enrolling for coverage that takes effect Jan. 1.
"Can you guarantee no American will experience a gap in their health care?" Rep. Kevin Brady (R-Texas) asked the Medicare and Medicaid chief.
Those individuals can transfer to new coverage through their insurer or go on the federal exchange, Tavenner explained. Anyone facing a gap in coverage in January "can go to the call center today and we can help them," she said.
During a news conference Tuesday afternoon, Julie Bataille, a spokeswoman for the Centers for Medicare and Medicaid Services, said, "We are confident that consumers will be able to enroll in coverage by December 15th for coverage in January."