Routes using HD terrain are limited to 16 different textures per baseboard but each baseboard can have 16 totally different textures. If a baseboard exceeds that number then the extra textures in that baseboard only will be replaced by one of the 16 textures already used. This is a limitation imposed by the hardware, not the Trainz software. According to reports most other games/sims that use HD terrain have a similar (or even lower) restriction. Unreal Engine, for example, limits you to 8 textures for what is their equivalent of Trainz baseboards. Others don't impose a limit but rely on developers to identify and sort out any problems (e.g. performance and/or visual issues) caused by the use of excessive textures.
I have only encountered this is one large route I upgraded from 10m resolution to HD. The upgrade reported that one baseboard had exceeded the 16 texture limit and it gave the co-ordinates of the baseboard so I was able to check it but could not see anything amiss.
For "normal" (i.e. non-HD) routes I believe that there is a limit to the number of textures that can be used. It is a large number that I have never remotely approached in any of my routes so I have never worried about it.
