How much unusable fuel is in the A320?
You can find the amount of unusable fuel in the EASA Type Certificate Data Sheet. Here is an example table for the A320-200 from that document:



In general, the cockpit displays will always show the amount of usable fuel only. This is mandated e.g. by EASA CS 25.1337:


  (b) Fuel quantity indicator. There must be means to indicate to the flight-crew members, the
  quantity, in litres, (gallons), or equivalent units, of usable fuel in each tank during flight. In
  addition –
  
  
  Each fuel quantity indicator must be calibrated to read ‘zero’ during level flight when the
  quantity of fuel remaining in the tank is equal to the unusable fuel supply determined
  under CS 25.959;
  Tanks with interconnected outlets and airspaces may be treated as one tank and need
  not have separate indicators; and
  Each exposed sight gauge, used as a fuel quantity indicator, must be protected against
  damage.
  


(EASA Easy Access Rules for Large Aeroplanes)