Washington currently utilizes 6 area codes. When first established by AT&T and the Bell System in 1947, 206 covered the entire state of Washington. The most recently added was 564 in 2017 as an overlay to 360. Today, 360 is the most populated with an estimated 3.6 million of the state's 6.7 million residents.
Sources: North American Numbering Plan Administration (NANPA), telecom providers, and the U.S. Census Bureau.