There are several reasons why enrollment numbers are important for the overall success of the law.
Most people spend relatively modest amounts on medical care each year, and a small proportion of patients accounts for the overwhelming majority of costs. Since older, sicker people are expected to enroll as the law lifts barriers that now keep them from getting insurance, premiums from lots of younger, healthier people are needed to help offset those costs.
Also, state numbers are as important as national totals. That's because each state's insurance market will remain separate under the law. "Obamacare" doesn't create a one-size-fits-all national program — like Medicare — but a bunch of state programs. That means lots of young healthy people signing up in California, for example, cannot cross-subsidize older, sicker people in another state.
"You can bust through these targets, but if it's mostly older and sicker people, then you are not in good shape," said Larry Levitt, a health insurance expert with the nonpartisan Kaiser Family Foundation.