According to a recent report:

• About 11% of all Medicare beneficiaries (5.6 million) receive their healthcare from Accountable Care Organizations under Medicare’s ACO programs
• About 35 million non-Medicare patients also receive care from these ACOs
•  Between 15% and 17% of the U.S. population (49 to 59 million) are served by ACOs

Source: "Accountable Care Organizations now serve between 15 and 17 percent of the United States, according to new research from Oliver Wyman," Oliver Wyman Press Release, April 22, 2015, http://www.oliverwyman.com/who-we-are/press-releases/2015/accountable-care-organizations-now-serve-between-15-and-17-perce.html