+1 for navidrome.
I’ve had better luck with the play:Sub app (iOS).
I think it’s important that these servers use a common API (subsonic), but it seems like the slickest apps are always targeted to one specific backend (plexamp, finamp, prism music).
If one of the copies is local and another is through an online backup, it's going to be a different in almost every way from local copies so you get the "2" by any definition.
If all your backups are to a set of tapes or, say 3 hard drives, and you have a rotation to keep one tape offsite, every copy shares too many traits in common. A fair number of Mac people use Time Machine to do versioned backups to an external drive (or drive array) and use separate software to simply mirror their drive to external drives, periodically swapping the onsite and offsite mirrored drives. To me, physically moving drives around is hardcore, I don't trust anything not automated, but it seems to be not that rare. I'm fairly lazy but my machine is a work laptop so I have a backup drive in the office, one at home, and have a cloud backup service so I have 3-2-1 without much thought or effort.
If backups are solely on hard drives, it's probably best to not use the same make/model purchased at the same time, for fear that they'll all fail within the same time frame.
The idea behind '2' is to avoid having all your backups dependent on a single point of failure.
For example you religiously have three copies of data but they all came from the same tape drive that silently went bad months ago and has been writing garbage. Or you recieved a batch of bad tapes etc...