There have been a few similar threads about this subject over the years, as you can imagine.
The tank (and lines) capacity is 18.5 US gallons (or 70 litres/liters, 15.4 imperial gallons)
The fuel warning light comes on (and stays on) when you have less than 2.75 US gallons in the tank. The light may come on before this, but that is due to the motion of the car and the fuel sloshing around in the tank, causing the sender to be exposed and activate the light. You should be able to drive (gently) around 50 miles (or more) before running out of fuel....
Therefore, a usual fill-up for a car that the light has come on consistently will be around 15.75 US gallons / 60 litres/liters / 13.1 imperial gallons.
The gauge isn't particularly accurate - it is simply an indicator of the approximate status of fuel in the tank - and is cautious so that you are unlikely to run out of fuel.