According to a Dan Mangan September 1, 2015 blog post for CNBC:
"In the first three months of 2015, the CDC said, the rate of people without health insurance stood at 9.2 percent. That's down from 11.5 percent in the same period in 2014, and represents a 20 percent reduction in the uninsured rate over the past year.In absolute numbers, 29 million people lacked health insurance in early 2015, compared with 36 million people in 2014. That number reached a high 48.6 million in 2010, the year the ACA was signed into law by President Barack Obama."
"The CDC's report is the latest survey that strongly suggests the ACA is behind a marked decrease in the uninsured rate in the U.S., as well as declines in the number of people who have difficulty accessing and paying for care."
Read more about how this decrease occurred