The White House expects 9.1 million people to be signed up for health insurance from government websites by March.That goal is much lower than a prior estimate of 13 million by the non-partisan Congressional Budget Office.
Last year’s launch of the federally-run website was plagued by technical problems.
Health Secretary Sylvia Burwell said the process would be a “positive experience” this year, but said some thing “will go wrong”.
“We will have outages. We will have down time,” Ms Burwell said, telling liberal think tank Center for American Progress she wanted to be transparent about her team’s expectations.
The Affordable Care Act is a huge overhaul of the US healthcare system, that aims to extend health insurance coverage to some of the estimated 15% of the US population who lacked it.
The enrolment period for 2015 begins on Saturday and will end in March.
The current number of people enrolled has been lowered due to more than 100,000 ineligible immigrants as well as people who stopped paying their insurance premiums, according to the Washington Post.
Ms Burwell said her agency expected about 83% of those who already have insurance through the sites would continue their coverage. They are included in the 9.1 million estimate.
But the law that created the exchanges was delivered another potential blow when the US high court agreed to hear a second challenge against it.
The Supreme Court previously ruled the US could impose a tax penalty for not buying insurance under certain circumstances.
But the high court agreed to hear a challenge over whether subsidies to buy coverage should be restricted to 14 marketplaces run by state governments.
The case could complicate enrolment by fanning uncertainty about who will be able to access subsidies after the court rules.