National Alliance for Public Charter Schools (PDF):

Across the country, more than 300 new charter public schools opened in the fall of 2016. Charter schools are public schools that have exibility to meet students’ unique needs, while being held accountable for advancing student achievement. Every year, the National Alliance for Public Charter Schools (National Alliance) collects data on the number of charter schools that opened and closed in each state that has operating charter schools. This information
is used to determine the current number of charter schools in each state, as well as to estimate total charter school enrollment at the state and national levels.

In 2016-17, there are more than 6,900 charter schools, enrolling an estimated 3.1 million students. Over the past 10 years, enrollment in charter schools has nearly tripled—from 1.2 million students in 2006-07 to an estimated 3.1 million in 2016-17. Between 2015-16 and 2016-17, estimated charter school enrollment increased by over 200,000 students. The estimated 7 percent growth in charter school enrollment between fall 2015 and fall 2016 demonstrates continued parental demand for high-quality educational options.