As a former Seattle missionary (2011), there aren't too many Mormons, but more than you would typically find East of the Mormon Corridor. I'd probably guess at a 2-5% Mormon population in the King County area, with a 1/3 activity rate. There's only one stake in Seattle itself, and half of that is made up of 3 singles wards based near UW, as well as a couple of Polynesian speaking wards that draw Tongans/Samoans from outside of the city.