Re: Which save files are safe for me to delete?

@NinaNinja2  Files with a number after .save, for example .save 2.ver 1, cannot be read by the game.  They typically happen when a cloud-storage service, for example OneDrive or iCloud, syncs the contents of Documents.  Your cloud storage may already have a file with the name Slot_[number].save.ver1, and now there's a new, distinct file with the same name ready to be synced as well, so the service renumbers one of the files.  The duplicate files may then be redownloaded to your device as part of the syncing process.

Because the game can't read any saves with nonstandard file names, you won't have been seeing them in-game, so you can delete them without having any practical effect.  Still, it's a good idea to make sure you have all the saves you're expecting to see before trashing any files; if you discover something missing, you can go through the extra files looking for it.  All you'd need to do to load one of these saves is rename it so it follows the usual standard.

Alternately, you could move all your saves except the ones you're playing now to an external drive and just let them sit there.  If you never need them, great; if you do, you'll still have everything you might want to use.  It's a good idea to back up your saves periodically anyway—you never know when something might go wrong.

Personally, for a given save, I make many manual backups and delete most of them when I'm sure I don't need them anymore, keeping the last 4-6 plus the occasional extra around a major sim milestone.  But there's no single best way.  Whatever system works for you is fine.