^ agreed.
You put as many as you need to reduce the bubbles. That may be 2, that may be 5 or 8 even. It just depends on how you have them set up, the sections of your sump, the flow of your sump, and the bubbles produced. Most people place 3 after the skimmer to reduce the micro bubbles created from the skimmer, becuase for the most part, 3 gets the job done.