I think rarity is but a word. Say fighters are common then fighter characters are common fighters. Say wizards are rare, wizard characters are rare wizards. The difference being that nameless fighters are assumed to exist but only named wizards actually do. If the party has one wizard and the arch-nemesis of the group is a wizards those two are the only wizards in the world. Unless of course the DM specifically introduces another one.
This reminds me of the word rare attached to the word Jedi in Star Wars campaigns.
Supposedly, Darth Vader and the Emperor through order 66 killed all the Jedi but it seems like every book post that point has a few Jedi that escaped the order 66 until it now seems like hundreds if not thousands of Jedi avoided being killed off.
Supposedly with the Sith they only have one apprentice but it seems like the Emperor had dozens running around.
Athas was supposed to have only 1 Dragon which then some author killed off and now there are dozens of dragons.
Dragonlance, ugh, that is so messed up now if you try to follow all the published books to the 'current' history.
Players, GMs, and authors will constantly add in and change the 'rules of rarity'.
If a GM says there are only the wizard in the party and the arch-nemis and the wizard in the party dies? Hey, guess who was just in the town over, the arch-nemis's other wizard that agrees to join the party and 'replace' the dead wizard.
Oh, someone got tired of playing the thief in the group and wants to play a wizard now. I guess are new replacement wizard has a brother/cousin that trained with him, ahh, ahh, in secret so no one knew there were actually now three, yes three wizards opposing the arch nemis of the rare wizard and one is dead now.
Rarity is a very silly word and thing to try and impose on a RPG story.