Above The Clouds, whilst that may be true for fixed wing*, the helicopter (EC135) in the Clutha crash had only two capacitance sensors in the main tank and one in each of the engine feed tanks (the red warning sensors are heated wires which only show if they are in the fuel or ullage). See the tank diagram on the page linked below:
http://www.pprune.org/rotorheads/528...w-pub-148.html
*Fixed wing aircraft often need multiple sensors in the wing tanks due to dihedral and the wing section leaving sensors fully submerged or fully dry. Three partially immersed sensors are needed to show a fuel surface plane, from which a fuel volume can be computed.