I would guess that this is between 1/2 and 1/3 of our North American TT car fleet, and around 2/9 of our total TT rolling stock fleet.
Here is 58 cars, of them, of various makes, including HP Products, Star Line, Joma, Possum Valley Models, Allegheny Scale Models, Gold Coast, and quite a few scratch-built ones.

The car body on the lower right of the above photo is the beginnings of a mid to late 1800's boxcar.
The entire bottom row is cars that James built here (I helped with the two brown ones).
There are some Maisto cars included in this, as they will be converted/modified into TT scale eventually, as we get more trucks and couplers.

Does anyone else have any North American TT Fleet Photos that they would like to share?