Depends on a lot of things:
access, wiring, fascia type, etc.
9ft of 2ft channel letters is what, like 4 letters?
also are they LED's or Neon?
LED's - are they self contained?
If so - Qty: 1 - 3/8" hole through wall per letter
Neon - Take Faces Off - Take Neon Out - Qty: 2 - 1 5/8" holes through wall per letter - Neon Back In Faces Back on
This is all assuming they are individual letters and not on a raceway.
What is the wall made of? Dryvit over wood.....or is there a chance of steal beams?
Plus you will have to get behind the wall to do all of the wiring. There are a bunch of variables to doing a channel letter install.