It's about 7 percent, there were 1.8 million people in King County in 2004 estimated by the Census bureau.
In the Seattle Metro area (including Pierce & Snohomish Counties & Bremerton as well), the total population was 3.5 million, good for the 13th largest city in the US, 3rd largest out west behind LA & the Bay.
This is good news, the more people the more important we will be, the greater likelihood one of our beloved rappers might actually blow.