I have a 400ex and i just got a e-series s-bend silencer for it and what is the best number of disces to run with stock jetting or stage 1 from the k&n power kit??

 

Put about 12 or 13 in. Change your piolet to a 42 and your main to 155. To get the full power it needs to get some more air. If you can run without the airbox lid or raise the front of the lip by pushing the locking tabs in front before closing the lid. This should get you very close.
This what I had before it was stolen last week.