The Hundredth Smurf
Bad luck invades the Smurf Village and 100 Smurfs are needed to dance a magical dance before good fortune will smile on them again. Vanity's new mirror is struck by lightning, creating a double image which serves as the 100th Smurf and everyone gets ready for the magic dance.
Because Harmony is forbidden to play in the Smurf's Symphony, Gargamel easily deceives him into playing the magic shazalakazoo.