Where can I find the maximum air speed for the current altitude in the 747-400 ?
It should be a red line/region on the Airspeed Tape indicator. I don't have FSX so I can't be specific. This feature was not correctly simulated in FS9, I hope they corrected it in FSX.
There is not a red line in air speed indicator. I can see it using many other aircrafts, but not in 747-400. I use FSX.
Have a look in the reference section of the kneeboard
(Menus: Aircraft --> Kneeboard --> Reference). It should state the max air speed.