You mentioned an article in the comments which appears to have the numbers you are looking for.
According to the article:
50 to 75 percent of the 14 million consumers who buy their insurance individually can expect to receive a “cancellation” letter or the equivalent over the next year because their existing policies don’t meet the standards mandated by the new health care law
So, that comes to 7 million to 10.5 million, or roughly 3% of the population.
I don't know if that makes for an 'official' estimate or not.
It should be noted, however, that these are 'cancelled policies due to non-compliance with ACA'. That's not quite the same thing as saying 'unable to keep their health insurance' as most people will be able to continue getting covered by the same doctors under the same insurance company under a new plan.
Obama is certainly getting some flak for this, as the administration was fairly adamant about the 'you can keep your insurance if you like it' statement.
On a technical level, that was clearly incorrect, and a rather major mis-step in communication with the public.
On the other hand, these are non-compliant plans, which typically means they were catastrophic plans. Those that would argue the other side would point out that a catastrophic plan is a pretty poor plan to begin with.