B & B wanting passports 5 months in advance...

that's normal; your question has come often on this forum:
https://community.ricksteves.com/travel-forum/italy/renting-an-apartment-asked-for-copy-of-passport

It seems to me that the more information you put out there loose in public, the better your chances for identity theft. If you let them copy your passport in their office, you know it's right there. If you mail them a copy 5 months earlier, who knows where it goes in the time until you get there.

In twenty years of staying in Germany, Austria, and Czechia, at forty-eighty different establishments, I have never been asked for my passport. A few times, in resort areas with kur taxes, I have had to fill out a form with my passport number, but that has been it.

No that is NOT normal.

It IS normal to ask to see (and copy) your passport upon checking in, but never in advance.

You don't even to have a passport 5 months before you travel.

I have never been asked, only on arrival.

We have ALWAYS been asked to provide our passports for copying when checking into Italian hotels.

And as passports are good for quite a number of years, persons who have previously traveled outside of the U.S. usually do have them in their possession 5 months before a trip.

As hotels are not common in the CT, it's probable that this is an apartment, and that flat owners like to officially register their guests further in advance than hotel staff. There are any number of reasons why, including the possibility of someone other than the owner handling turnover of keys, no formal office or equipment for making the necessary copies upon arrival, no human at all handling check-in (entry is via a lockbox with a code provided after the rent has been pre-paid), etc.

No, there's almost nothing on them that isn't on a U.S. driver's license so those copies don't bother us.

The difference is that this is Italy and Italy has different laws and customs than Germany, Austria, and Czechia.

They need some details they must forward to the cops, they do not need a copy of the passport 5 months in advance.

I have been to Italy 5 times for 2-3 weeks each trip. I have used different types of booking reservations. Booked directly with hotels and B&B, booking.com and Airbnb. Never was asked for passport info until I arrived at the accommodations. So in my many nights in Italy not asking in advance not normal it is very strange.

As has been mentioned it's a legal requirement that they collect the information and forward it to the local police, though that usually happens during check-in.
Per Kathy, my guess is that the request was probably forwarded to you early because you booked early, and it's just one of the administrative items on their list.

More from the RS archives:

https://community.ricksteves.com/travel-forum/general-europe/providing-proof-of-identity-when-renting-in-europe

And:
https://www.fodors.com/community/europe/common-practice-to-provide-passport-copies-for-apartment-rental-in-venice-999471/

If this a self check in lodging, they tend to ask for documents in advance, but I wouldn't provide 5 months in advance. Email that plans can change on either person's side andnuou would prefer to provide copies 1 or 2 weeks before when you touch base to reconfirm all lodgings, preferences and details.
Or waffle stating you may need to renew or replace uour passport so prefer to domit closer to check in.

“ If this a self check in lodging, they tend to ask for documents in advance, but I wouldn't provide 5 months in advance. Email that plans can change on either person's side andnuou would prefer to provide copies 1 or 2 weeks before when you touch base to reconfirm all lodgings, preferences and details.
Or waffle stating you may need to renew or replace uour passport so prefer to domit closer to check in.”

But why not just comply with the request? What is the risk?

Aussie, plans can change and many don't want to share passport information unless neccessary. Even in my lenient Canadian mind, 5 months is too soon. I was just offering an alternative.

Have you also paid a deposit? If not, and you are going to pay on arrival some places ask for some official 'details' such as a photocopy of a document just to check you are serious about the booking, not because they intend to do anything with the information at that point. In Italy all accommodation establishments have to report guest details to the police when they arrive. Since the details required are different according to whether the guests are Italian or other nationalities sometimes it's just easier to ask for an ID copy to be sure to have everything.
As to what to do, often it's easier to tell the truth - reply saying you're not happy about it and what are the alternatives.