Besides the new-ish Alakazam ex, Pokémon can't deal damage from the Bench unless it's via Ability (Inteleon's Quick Shooting, for example).
If an attack doesn't have a number denoted next to it, that means it either does no damage or its damage is determined by its effect. In Clefairy's case, Mini-Metronome would do however much damage the move you choose of your opponent's Active Pokémon would do (if any), assuming you get heads. You have to follow all the instructions of the chosen attack though. If you're unable to perform all of the instructions, such as discarding Energy, you won't be able to use that attack.
Also, attacks without a number next to them will sometimes say something like "Put 2 damage counters on your opponent's Active Pokémon." This isn't the same as doing 20 damage to your opponent's Active Pokémon since you wouldn't be applying Weakness/Resistance.