Great question. I've been using 9 bags per gallon as well, but the Big Book of Kombucha recipe is for 4-6 bags. I just made a batch on Saturday with 7 bags per gallon to see how it comes out.
I emptied out a few different tea bags and they all weighed 2 grams. My tea balls hold 12-14 grams (the gun powder green tea rally expands) and my recipe is 12g green tea and 14g black tea in 2 gal. The important thing is I can still taste the tea flavor in my finished booch!!