Tennessee currently utilizes 7 area codes. When first established by AT&T and the Bell System in 1947, 901 covered the entire state of Tennessee. The most recently added was 629 in 2015 as an overlay to 615. Today, 615 is the most populated with an estimated 1.6 million of the state's 6.3 million residents.
Sources: North American Numbering Plan Administration (NANPA), telecom providers, and the U.S. Census Bureau.