The Ehrenhofer bus was built by Union City buses in Indiana. It is also an adult bus as there was a difference between a city bus (like Ken's) and a school bus.
There were 48 Union City adult buses built between Nov of 31 and Feb of 32. I believe this is the third rarest bodystyle of AA trucks.
https://aafords.com/closed-bodies/
Many other independent body makers would have built their own bus bodies on a Ford chassis. Greenfield Village has a Union City Adult bus. The other buses I know of (huff n puff, one out of New Jersey) were not Union City.
Fun fact. The Union City adult bus came from the factory with two batteries and the low speed "police" generator.
If you're only looking to see how many Union City Adult buses remain thats a very low number. If you're looking to see how many AA buses of any type exist that number will be much higher and could include some Frankenstein or more modern creations.
-Tim