Mine has just over 300 tags. My rule is that each book gets one tag, and that's based on what the main genre is. I know, there's a ton of books that straddle the fence, but I go with that the genre the book mostly fits. Then I break that subject down, into groups that I'm interested in. For Scifi, my tag list looks like...
Scifi (which holds the general group that isn't any of the below.)
Scifi.AI
Scifi.Anth
Scifi.Apocalyptic
Scifi.Cyberpunk
Scifi.Dystopian
Scifi.Fant (For those Scifi that include magic)
Scifi.FirstCon (First Contact)
Scifi.Hard
Scifi.Horror
Scifi.LitRPG (where the gamers get trapped in their gaming worlds)
Scifi.Military
Scifi.SpaceOpera
Scifi.Tech (future stories, that deal with technical advancement, not so much space stuff)
Scifi.Thrill
Scifi.Time (Time travel)
Scifi.West (Firefly-like stories, loveable rogue outlaws with a Western vibe)
Scifi.Zombie
and each of those tags can be broken down into .Anth and/or .MM.
(Anthology and/ or lesbian or gay stories (as in fanfic's label of M/M)
I save all my books to the hard drive, in those files in epub format, and never touch anything in the Calibre library files. This way, if I want to sideload a bunch of books with a certain tag, they're already sorted by what I'm in the mood for.
The mysteries have even more sub-tags attached.
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