Ulimit setting for Mongo DB

Ulimit setting for Mongo DB

I found the normal ULimit setting for mongo db is as follows:
-f (file size): unlimited
-t (cpu time): unlimited
-v (virtual memory): unlimited [1]
-n (open files): 64000
-m (memory size): unlimited [1]
-u (processes/threads): 32000
Just wondering why the "-u", recommendation is such high..
How many process do mongo generally spawn for each CRUD operations?