Hi Alessandro
I had my formal inspection by Turismo a week ago (I'm in Andalusia so it may be different for you) and I asked them that same question! Their answer, finally, came down to "anywhere"! I put this at the end of "Other things to note."
"We are fully compliant with Turismo de Andalucia requirements for holiday rentals. Our number is VFT/MA/02644"
You're right though that most listings don't show it. In Andalucia the regulations only came into force this May so I don't think people have got used to it yet.