According to this estimate[0] based on viral mutations, there were 600 cases in Seattle on March 4th. That puts the multiplier at about 20x cases per confirmed case. With an estimated doubling time of 7 days[1], that's about one month worth of spreading.

(Disclaimer: all numbers approximate, high uncertainty everywhere.)

[0] https://bedford.io/blog/ncov-cryptic-transmission/ [1] https://bedford.io/projects/ncov-phylodynamics/