We had a 33 1/2ft Carver Mariner and yes, I know it's a boat & not a bus. The reason I brought it up is such that when we had an inspection by the Coast Guard for certification we were told that due to the size we would be required to have 2 fire extinguishers. I feel if 2 makes the Coast Guard happy then 2 should be sufficient.
I would think that 2 should also cut it for a bus. Ours is a 35 footer. We plan to have 3, only because 1. our engine is inside the bus, 2. Our galley is situated closer to the front than the rear & 3. Our bedroom is isolated from the galley and whoops I guess there's a 4. It would make Momma happy.