The typical transit bus is 40 ft. suggesting 40 passengers if only seats are used and 70 if folks are standing too. He counted 146 large transits. The total capacity of the public transit buses (just the transit!) in the New Orleans photos is between 5,840 and 10,220 people. He also calculated they would each have enough fuel for a trip to Houston with fuel to spare. On the school buses, he counted around 246 buses and not assuming maximum capacity of 3 children per seat but rather only 50 adults per bus, he calculated a capacity to move 12,300 adults with only seats utilized. However, these buses may have fuel issues for a long haul. CNG (Natural Gas) requires close to 1 hr for refueling and such buses are comparable to 4 miles per gallon. Refueling might be very difficult if CNG isn't readily available en route. Diesel would be better with slightly better fuel efficiency and far less fuel time as well as being more readily available.
Thus the two major bus fleets at the mayor's disposal could've moved 18,000 to 22,500 people from New Orleans WITHOUT maximizing transport capacity.