It takes less than 10,000 btu to maintain a boil with 10 gallons, the bigger issue is how fast you can get the liquid to a boil. The more Btus you have, the faster you can bring the wort to a boil.
Lot's of people boil with 3500W electric elements. That converts to about 12,000 Btu. Of course, the electrical elements are near 100% efficient. Propane burners lose significant heat to the surroundings.
If you want to get away with as little Btus as possible, I'd highly recommend insulating your kettle with some high temp insulation and building a permanent wind screen for the burner.