Obama's health care can co-exist with private insurance?

A recent study says the public health care plan can co-exist with private insurance because they estimate only 12 million people will sign up. The statistic that Obama used in his speech is that 40 million people are uninsured. If only 12 million sign up, what about the other 28 million? Does it have to p to cover the 40 million? Is it OK for it to p because only 12 million will use it? Are any of the projections accurate given the wide gap in numbers? Are they just making up numbers to fit present argument? It doesn't add up.