The republican leadership is trying to rush the Graham-Cassidy bill through the Senate so quickly that the Congressional Budget Office will be unable to do its normal, full-scale analysis of the bill. So, by the time the Senate votes next Wednesday, the CBO won't be able to tell us how many people would have health insurance under the bill (as compared to under current law).
But the Brookings Institution has stepped up and done an analysis. It estimates conservatively that, under Graham-Cassidy, 15 million people would lose coverage over the next two years. After 2019, the number of people without health insurance rises steadily over the next 8 years. By 2027, 32 million fewer people will have health insurance. [See the chart at the end of this post for more details.]
Brookings acknowledges that there's considerable uncertainty in these numbers. But again, the numbers are conservative estimates. For instance, Brookings says that the coverage reduction in the next two years will be larger than 15 million "if uncertainty about the effects of the more radical changes implemented by the legislation in 2020 caused some insurers to pre-emptively withdraw from the individual market." The large coverage reductions in 2020 and beyond don't include the potential for turmoil in the insurance markets caused by earlier changes, nor do they account for changes in the Medicaid program (on top of repeal of Medicaid expansion) that may reduce coverage even further.
As you may know, Graham-Cassidy repeals the Affordable Care Act's Medicaid expansion and subsidies for people who cannot afford premiums, and then sends money to the states in block grants. In that regard, Brookings reports:
A long literature in economics has examined how state and local governments adjust their spending in response to the availability of targeted federal block grant funding. That literature has found that states often do divert a large fraction of block grant funding to unrelated uses, although the extent of diversion varies widely from setting to setting, from near complete diversion in some settings to virtually no diversion in other settings. The possibility that the Graham-Cassidy funding will be diverted to unrelated purposes thus has ample precedent in prior experience.