The original 30+ million uninsured included Medicaid, Medicare and Chip eligible persons who were not enrolled, but still technically covered if anything happened. There were also a very large number of financially secure people who chose not to ensure. The actual number of involuntary uninsured was somewhere in the teens to low 20's (though some put that number even lower).
The predictions for the rate of uninsured, after the ACA, by the CBO itself, is almost identical, 30million plus.
The Medicare, Medicaid and Chip users should now be registered but the people who choose not to buy are still likely not to (the fine is unenforceable and a pittance compared to most yearly plan rates). But now you can add the relative poor, who even with subsidies cannot afford the increased plan prices.
There is also a new group, those who were insured but can now play the insurance lotto. Since they cannot be denied coverage when needed, if they're willing to pay out of pocket for the waiting period between enrollment and coverage, they can simply cancel their plans and wait to see if anything catastrophic happens, then enroll. And by catastrophic I mean cancer not broken bone (which can often cost less to pay cash for than going through insurance anyway). So this person can save thousands a year and if something unforeseeable happens, pay their medical expenses for a couple months until their new policy takes affect. If they put even half of their normal premiums into a savings account, in most cases this would be a financial win for them.