The number of keys a MIDI keyboard should have depends entirely on how you plan to use it. A full-size piano has 88 keys, or seven octaves and an additional three keys below bottom C. If you are a piano player or want a MIDI keyboard to learn to play piano, this is what you should be aiming for. Most synthesizers have 61 keys or five octaves, making this something of a standard MIDI keyboard size for electronic musicians and producers. Small and portable MIDI keyboards can go down to as low as 25 keys or two octaves. While this is fine for a portable device, it could be restrictive for day-to-day use, especially if you like to play two-handed chords.