Modern fuel pumps are cooled by the gas.
Modern fuel systems are closed looped systems.
Fuel is continuously pumped to teh engine, more fuel than the engine can possible use, the excess is returned to teh tank.
Running low on fuel repeatedly WILL shorten the life of your fuel pump. Low fuel will get hotter than a full tank, especially in teh summer. That heat hardens the seals in the pump, which casuses tehm to leak, and eventually fail. Heat is also the number one reasons transmisions fail. In hte case of the transmission the heat is generated by shifting gears, hence the recommendation to downshift a gear if your transmission is hunting for gears. Yes your gas mileage will reduce, however your transmission will last longer.
As to the original question. How much is left all depends on your fuel sensor, which is part of your fuel pump. Each one reports fuel level differently. My truck holds 17.5 gallons. My Low Fuel light comes on when I have about 3 gallons of gas left. If Im city driving I have about 45 miles before I need gas.