Coming from What franchises are these weapons from?
I can't help but wonder.. Why does Finn have so many swords?
Looking for the in-universe answer.
Most of Finn's swords come from looting and his various adventures. The tree fort has a whole room dedicated to treasures found from their adventures, it's not hard to imagine that they would find swords in any of the dungeons that they explored. The swords of most importance are the root sword which he found in the episode 'mystery train', the demon blood sword (a gift from his father, later destroyed) in the episode 'dad's dungeon', the grass sword 'blade of grass' (a replacement he brought when he lost the demon blood sword, later lost), and the finn sword first seen in the episode 'is that you?' (gifted to him on a quest). A full list of his swords can be found on the wiki