On Thursday, the Department of Health and Human Services announced that 6 million Americans had now selected an insurance plan through President Obama's health care law due to a late surge ahead of the advertised deadline of March 31 to sign up for coverage without facing a penalty. But that leaves a number of unanswered questions:

1. How many people have enrolled in Obamacare?

The 6 million figure touted by the administration refers to the number of Americans who had signed up a plan through one of the program's exchanges. But to complete enrollment, individuals have to consistently pay their premiums. The administration has not released data on how many have paid, though industry sources have said that 20 percent to 25 percent have not kept up with payments. If true, that would put actual enrollment in the mid-to-high 4 million range.

2. How many of those who have enrolled in Obamacare were previously uninsured?

The central goal of Obamacare was to expand insurance coverage. But changes in the law also meant that millions of Americans received cancellation letters from their insurers. What remains unclear is how many of the 6 million Americans HHS officials say have selected a plan wouldn't have been insured without Obamacare. Put another way, the effective net gain in insurance coverage is unknown.

3. How many of those who are enrolled are young and healthy?

Before the opening of the exchanges, the Obama administration was telling reporters that for the exchanges to be viable, nearly 40 percent of enrollees would have to be young and healthy. This demographic is necessary to offset the costs of covering older and sicker patients, particularly those with pre-existing conditions. Through February, just 25 percent of enrollees were in the target 18- to 34-year-old demographic. HHS has long insisted that younger and healthier Americans were more likely to wait until the last minute to sign up for coverage, so when the next enrollment report comes out, this will be a closely watched metric. It should be noted, however, that age data itself is a crude metric, because it's perfectly possible that somebody could be older and healthy, or young and sick.

4. How are individual states doing?

Obamacare can't be thought of as one national risk pool, but as 51 different risk pools. For it to succeed, there have to be viable exchanges in all 50 states, plus the District of Columbia. But as I've written, through February, there was still a wide variation in how individual states performed. When the next enrollment report comes out, it will be important to look at whether the gains in March were concentrated in states that were already doing well in signing people up, or if the lagging states were starting to catch up to their enrollment targets.

5. How long will open enrollment actually last and how many more people will sign up after March 31?

Open enrollment was supposed to end March 31, and, according to the administration, it still will. But in reality, HHS announced a policy that would allow anybody who has attempted to buy coverage prior to March 31 to sign up beyond that date. Because all that's required is for individuals to personally attest to the fact that they tried to sign up — and there is no attempt to verify this and no consequences for those caught lying — the enrollment period is effectively delayed indefinitely. It's unclear how long this special extension will last, how many additional signups it will yield, and how the demographic breakdown will work out. In the coming months, insurers will have to submit premiums for 2015, and this will require knowledge of what the risk pool on the exchanges looks like. That becomes more complicated if it's a moving target.