According to SDOT:

There are an estimated 500,000 on-street parking spaces in Seattle.

There are 11,900 paid on-street parking spaces (metered) in Seattle, nearly 3,000 more (a 32 percent increase) than there were 10 years ago. SDOT did not have the percent change in off-street parking (garages).

According to census data, Seattle's population has grown by roughly 10 percent over the same period.