Depends on manufacturer
Approx 10 percent of the fuel tank capacity. If 20 gallon tank, then 2 gallons.
70 Liter, or 18.5 Gal., or 15.4 Imp. Gal.
because it's hooked up to a thing usually called the sender unit that's sitting in the tank. The sender unit usually have some sort of float which can follow the fuel level.
Full gas tank capacity is 19.2 US Gallons. You do the math.