The maximum altitude will be different from map to map. For players, it's pretty much determined by the boundary volume (OOB grid). We've deliberately made the upper reaches of the OOB grid much higher than any player is likely to reach, with the exception of the arena-style maps.
There's also a vehicle flight ceiling, which is usually far lower than the upper reaches of the OOB grid. The flight ceiling is set differently again for each map and is measured in units above the map's origin (0,0,0 co-ordinates).
I know I've already mentioned all this in another thread, but I can't find it...