Sadly, we can't have a lot of textures within one titleset, just like in wc3, but I figured out how to customise them at least:
Go to your Data Editor and open the "Texture Set" Tab.
Now find your texture set (ex: Bel'Shir)
Now find the "Textures - Blend" and open it.
There you can customise the specific title set.
(Restart the editor after changing it)
EDIT:
It IS unlimited. After changing it back to what it was, the other texture which I placed on my map (from the different titleset - which I now removed from the titleset) was still there. By adding/removing it, you can use everything.
If it helps, you can also add additional cliff tiles. I've been able to use three different cliff types in the one map, and you could probably add more.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.