In my experience 9 or 10 dice is a good number. I don’t know that there are really hard limits, as it mostly just shifts the results of the larger rooms with larger numbers. I wouldn’t go significantly higher though, at least not without modifying it to split up Halls and Towers. Alternatively just roll handfuls separately, assemble, then connect the results.
That matches my intuition. Smalller numbers are good to start with. I think I tried 4,5 then 6 dice just to feel my way into the system. It started to feel good when I got to 8, 9, 10. Thank you for such an entertaining little game. I now need to make a bit more time to start drawing what the dice reveal.