How are IIS7 threads assigned?

I added log4net to my application and can now see the thread Ids of user activities as they navigate through my website. Is there any specific algorithm to how threads assignment happens with IIS7, or is it just a random number assignment (I suspect it's not completely random because my low traffic site show threads mostly in the range 10-30)? Any maximum to the number of threads available? And I notice that my scheduler shows up with a weird threads id -- any reason for this? The scheduler is Quartz.net and the id shows as "Scheduler_Worker-10", and not just a number.