Book for 1 person and arrive in 5 people, it is not correct and is not polite.
Bedding configuration, linen and towels.. it is completely different. Can you immagine to invite one person for the dinner and he shows up with 5 people !!
BTW. the city tax in Rome is 3.5 euro per person per night. and it is listed in the house rules on the site: The overnight tax is a fee that will be asked to anyone staying in every overnight accommodation inside Rome’s boundaries. The overnight tax is : 3,5 euros per night per person, with the exception of children up to 10 years. The fee, isn't included in the total rate, must be paid cash at check in.