I ran mine out on purpose when it was fairly new to find out how much reserve it had. Of course I carried extra fuel with me. It does hold 19.5 gallons but you can get slightly more in it if you push it. I understand overfilling is not recommended because it can damage the evaporative emissions system.
Originally, I had about 3 gallons reserve when the light came on. After I changed the fuel pump with a new sending unit, it now has only about 2 gallons reserve. Those number vary depending on if I am driving around town and how hard I a braking or driving on flat highway.
2002 FX4 Bright Red Supercab 4.0 SOHC, manual 5-spd & Tcase, 33" BFG MTs, K&N air filter, Modified air box inlet, Gibson SS cat. back exhaust, Bama flash, Hidden Hitch round receiver, Rear helper air springs, Electric trailer brake controller, Line-X bed liner, WAAG bar with KC lights and Wildernest shell.