As far as I've ever known max motoring is as high an N2 you can get. When N2 stabilizes and is no longer rising then you are at max motoring.
I have seen many diffrent max motor levels on the same aircraft type with the same engines. It all depends on engine condition,age, windspeed and other variables.
I don't know if there is an exact number but it would be hard to keep constant, and would be a pain for us engineers to diagnose a snag that was written up as "did not reach max motoring".
Hope this helps.