The Seattle Sounders have advanced to the Major League Soccer Cup the last two seasons, winning in 2017, the year after the Portland Timbers won the MLS Cup.
The Sounders averaged 43,666 fans per game last year while the (relatively) nearby Vancouver Whitecaps and Portland Timbers average over 21,000 apiece. The New England Revolution averaged 19,367 fans a game.
https://en.wikipedia.org/wiki/Major_League_Soccer_attendance
I am a baseball fan who keeps promising to attend soccer matches. When my wife was out of town last fall I wandered down to a Portland Timbers game and paid a scalper $20 for a ticket that turned out to be void. I've dealt with scalpers my entire life and that's the first time I have been burned. I shrugged my shoulders and wandered home.