Rock Band is at its best when you have one or two other people available to form your own band. However, solo artists will also find challenging gameplay good for hours of fun.
Both single player and multiplayer modes are available in the game. In multiplayer, you can have any two instruments played or with enough pals and an extra guitar controller from Guitar Hero 1 or 2 you can get a full quartet going with drums, bass, lead guitar and vocalist. I would assume if you are coordinated enough you could also play an instrument and sing allowing for the strange notion of single player play in multiplayer mode.