Just one but is must have a clear view and cannot be mounted inside of the boat. Mine is mounted on the bottom of my jack plate. I lose SI when on plane but wouldn't use it then anyway. When on plane I have the map screen with a little bit of down sonar. These 3 youtube vids really are worth your time to watch if your in the market for a new unit.
I can tell the difference between soft and reef bottoms about 40-50 yards out of each side of the boat.