I don't have hard data but it seems like there are an awful lot of venues putting on music in the Seattle area, way more than I can remember for the last 40 years, since I was old enough to notice. Seattle is a little like Nashville, a smaller city that has an outsize music industry, not on the scale of New York or LA but around the 3-5 biggest city range (3. Chicago, 4. Houston, 5. Philadelphia) and certainly way larger than #6 Phoenix with less than half the population.
I blogged about this, there are free shows all over, especially during the Summer.
Kenmore has free concerts Thursday nights, Out to Lunch is every Wednesday and Friday, Bothell has multiple free venues including the Lyon's Den coffee shop and the city's free concerts in the park series. The merchants along the Bothell-Everett Highway throw Friday night/weekend barbecues in some of the strip malls that have room, hiring local bands for free music. Edmonds and Shoreline all have regular free concerts, and you can work your way around Lake Washington with shows in Kirkland, Bellevue, Renton, Kent (oops, veered away from the lake), South Seattle, West Seattle, Downtown Seattle, Capitol Hill and several north end venues and so on towards Everett and Tacoma and beyond. Keep in mind I'm just talking about the free shows here that book assorted local bands, and I'm sure I'm missing many (the venue in Magnuson Park by the community garden that feels sort of hidden away is a personal favorite).
There's also organized musical groups like choirs, instrument themed clubs (horn groups, mostly), orchestras and bands all over. The school system uses bands and orchestras them from grade school up to high school and college, with some like Garfield and Roosevelt high schools known nationally for their jazz bands. My mom took me to a concert by the South Snohomish County Brass Band or Club or something like that and they played a couple hours of large and small ensemble pieces extremely well including the military's signature songs and that piece they use in Monty Python's Flying Circus that ends with a big raspberry - they left the raspberry out, though. The Seattle Symphony and the opera at McCaw hall are probably regional peaks for high brow music.
Many churches put on concerts of one sort or another; even small towns like Kenmore have at least 4 and probably more church based venues that have live performances on occasion. Services use music more often than not and peak services (Easter, Christmas) have elaborate musical productions with paid bands (horn sections, strings) and large choirs.
All of those venues - and I haven't even got to the bars yet.
The smaller towns like Kenmore have 4 or 6 bars with live music and Seattle has a few in every major neighborhood and large numbers in some areas like Capitol Hill and Downtown. There are probably more than two hundred bars with live performances on weekends around here; if you're willing to drive as far as Bellingham or Olympia there are so many it starts getting mind boggling - hundreds of venues, perhaps 800 to 1000 sets a night, 600 to 800 bands, around 2,000 performers and another 1,000+ to do sound, security, collect cover charges and sell drinks and so on.
Finally you get to the destination venues. While they may have a bar (or several) associated with them the bar usually only operates when the venue is in use for a show. There are getting to be a fair number of smaller ones like the Fremont Abbey and multipurpose buildings like the Wade James theater in Edmonds, performance venues at many if not most community colleges in the area, with multiple venues at many of the larger universities in the area. Medium sized regional venues are all over, Edmonds has it's 600 or 800 capacity performing arts center, Lynnwood has space at it's convention center, Everett has one of the larger (holds thousands) and several smaller, Kent has a good regional facility that holds thousands. The Paramount is one of the nicest, large and beautiful. The Moore and the Neptune are also quite nice; the number of options in Seattle alone just for mid sized destination venues is pretty sweet. , At the big end of this list are the 10,000+ venues: the Key Arena, the Tacoma Dome, outdoor venues like Memorial Stadium, Whitewater Amphitheater, The Gorge, sports stadiums, the list is surprisingly long and I'm sure I'm missing plenty.
The Seattle Center contains a cluster of venues (2+ in the EMP, the stage in the Center House, 3 or 4 different playhouses, the opera house, the exhibition hall, the Vera Project, laserium in the PSC, during the summer another 5 or 6 outdoor venues) including the Key and Memorial stadium, both easily able to handle crowds well over 10,000.
Bumbershoot is able to serve hundreds of thousands in 3 days and put on a staggering amount of music in one weekend.
The Seahawks and Mariners stadiums also host shows with 10,000+ attending a single performance. They don't do many shows, but they get a few in every year.
Think about how many slots need to be filled every night, especially on the weekends. Say 300 bands do gigs on a Friday, another 300 on Saturday, maybe 300 for all the rest of the week, we're closing in on 1,000 gigs a week. There are probably a few hundred bands that get work close to every week and a much larger pool that performs at most 1 or 2 times a month, there would have to be more than 1,000 bands in the 1-2/month category to fill the slots if my math is correct. That explains why most of the sets I see are bands I've never seen before: there's just so many local bands that even after 165 sets in 2011 repeats are still rare.
The math also leads you to conclude that there are something like 10,000 people in bands getting gigs at least every few months in the greater Seattle area. Very sketchy math, so there could be large discrepancies (in other words I could be way off, maybe only 1,000 or maybe 40,000, I'm extrapolating off of local observations) but it's most likely in that ball park.
Another 1,000+ are needed to do sound and lights and load-in & out and sell merch and tickets and book shows and provide security and so on.
You figure there's at least that many in bands that are forming and trying but not getting gigs, and that many again who (like me) maybe play an instrument or sing decently, were once in a band or choir but haven't been for years who play and sing at the occasional Christmas or Birthday party. Oh yeah, I left out house concerts to, there are many of those happening quietly all over the area. And street buskers.