How many Americans will gain insurance under health reform? Good question.

In the health policy world, you get used to seeing “32 million” a lot. That’s the additional number of people that the Congressional Budget Office estimates will gain insurance coverage under the health reform law.

It’s generally taken as fact--but it probably shouldn’t be. Two new studies published in Health Affairs this afternoon underscore how much wiggle room there is in those estimates, especially if the health reform law gets changed further.

The first study, from three Harvard researchers, looks at how many Americans will sign up for Medicaid, the entitlement program for low-income Americans. The CBO estimates that 16 million Americans will gain coverage through Medicaid. The Harvard researchers argue that this 16 million figure assumes only 55 percent of eligible recipients actually sign up. They come up with three scenarios for low-level, mid-level and high-level enrollment, that range from 8.5 million to 22.4 million new Medicaid recipients:

All this variance has huge cost implications; it means that the Medicaid expansion could cost anywhere from $34 billion to $98 billion by 2019. Where we land on that spectrum depends a lot on how aggressively the administration, states and non-profits are in getting the newly-eligible signed up for coverage. Some groups, like Enroll America, are already getting going on this.

The second Health Affairs study looks at what would happen if the health reform law’s insurance mandate were eliminated, an issue the Supreme Court could soon decide. Without a mandate, 23 million fewer people would buy insurance, according to the Lewin Group’s new analysis. This estimate is higher than that of the CBO and close to those of other researchers like MIT’s Jonathan Gruber. No mandate means that premiums would likely skyrocket as much as 27 percent, especially if insurers are still forbidden from taking preexisting conditions into account.

This is all to say that the CBO estimates, while helpful, are likely guideposts with a decent amount of wiggle room. Where we eventually land is likely to depend on a lot of factors that the CBO can’t weigh in on, and ones we’ll see unfold in the coming years.