The manual lists the reserve as 1 gallon. Fuel economy tends to differ from rider to rider. The most accurate way to figure out how far you can go on reserve would be to take a ride with a spare fuel can on board and see how far you can go before it runs dry.
Btw, your 19 kpl is pretty good, I'm getting just over 15 kpl on my winter city commute. I don't think I've seen much better than 17 kpl out touring.
2014 350 EXC-F
2006 Djebel 250XC