The number which keeps getting thrown around is "over 8 million" (which is true), while the official CHIP website lists it as 8.1 million (which was true for 2014).
However, out of curiousity, I took a look to see what the 2015 data is, and sure enough, it was up to 8.4 million last year.
Here's a table showing, state by state, how many children were enrolled in the CHIP program last year (as well as the number in Medicaid, which, surprisingly, is actually more than 4x higher). In fact, well over half of all Medicaid enrollees are children.
NOTE: "Ever Enrolled" is confusing; it makes it sound like this is cumulative since the 1990's. It actually means "ever enrolled during that year", since there's a lot of churn with children moving into or out of the program as their family situation changes over time:
CMS approves Arizona’s plan to re-open CHIP program
Today, the Centers for Medicare & Medicaid Services (CMS) announced that it has approved Arizona’s plan to allow new enrollment in the Children’s Health Insurance Program (CHIP) after enrollment was frozen for several years. Now all states provide CHIP coverage to eligible children.
“Today’s approval is a step forward for the health of Arizona children in low-income families,” said Vikki Wachino, CMS Deputy Administrator and Director of the Center for Medicaid and CHIP Services. “With Arizona’s decision, all states in the nation now provide CHIP coverage to any eligible child who applies. More children in Arizona will have access to coverage early in their lives, which helps kids grow into healthy adults and provides parents with the peace of mind that comes from their children having affordable coverage.”
Having coverage through CHIP improves children’s health and increases their ability to succeed in school. Recent research on Medicaid and CHIP shows that these gains are long lasting, with children who gained coverage experiencing better health, higher educational attainment, and higher earnings as adults.