AirBnb has no policy regarding the number of guests sharing a single shower. I recently stayed in an AirBnb in Rome from May 1-31st. 2025. The 1st 2 weeks 3 of us shared 1 shower. It was never cleaned. The 3rd week 3 more guests checked in. 6 people sharing 1 shower. It still was not cleaned. There was 6" of standing water and soap scum. Sunday morning I texted the Host and requested the shower be cleaned. Finally, Monday afternoon someone cleaned it, but it still did not drain. When I booked I was charged 30,00 for cleaning service.
Moral of the story, before booking ask how many rooms and how many showers. I am staying in an AirBnb now June 1-30th. with 2 guests/shower.