For the Westside Series their will only be one class of racers in different age groups. I havent done the Westside it starts early in the season so the interest isnt quite as great. I am pretty sure it attracts a fairly hardcore group of racers that will ride all year round. Hard to say what the attendance will be but my guess would by 80-120 racers, The guys and gals who run the Westside Series have a big jump on fitness when the Indie Series starts. The Indie Series will have around 275 riders spread across Novice, Sport and Expert classes. Each class has about the same number of riders. Everyone signs up for the class they think they fit best in. I think I raced in about 10 regular races from April until September. There are also some low key weekday races. During the summer on Wednsday Nights there is a race series called the Wednsday World Championships at Seatac. There are no winners or anything you just race and you can quit when ever you want. The promoter counts laps and I think at the end of the year he gives out awards for most laps completed.
Go check Seatac out, its a fun little place. You wont be able to pre-ride the exact-same course because they always change the course but if you ride it enough you get pretty familiar and some of the trails they always use in the same direction.