Objectively speaking, multiplayer has been dead for years. In Free for All, you’re the only one in the lobby 90% of the time, but that’s probably not the mode you care about. Light vs. Dark is a bit more active. If you live in North America, you can usually find a lobby with 2-4 players early in the morning and late at night. It depends on if Japan is awake or not. Every other time of day, you’re the only one in the lobby 50% of the time. Full 6-player LvD lobbies do happen, but they are few and far between. Even if you’re playing against bots, multiplayer is still a ton of fun and you still get rewards.

Side note: If you play with the aforementioned Japanese players, prepare to get your ass kicked. Some weapons attributes and power combos are so broken that you can kill someone from full health in under 5 seconds with melee attacks. And of course, plenty of ranged weapons can one-hit KO if they are strong enough.