All 28 duels occur as part of missions, including campaign, side missions, and mythic tales. These cannot be directly tracked with wind or discovered by golden bird or firefly.
Only 5 duels are tracked in your Collections screen, and these are only the 5 straw hats leading up to Kojiro during the "Six Blades of Kojiro" mythic tale during the second phase of the game.
There is a duel that ends the campaign, so this achievement can only be obtained at the soonest during the final mission where
Mono No Aware is earned.