I hear only one note when I play the keyboard

KORG iMS-20 is a monophonic synthesizer which plays only single notes. Most early analog synthesizers were monophonic. The original MS-20 was also a monophonic synthesizer. Although it cannot play chords, it allows performance techniques and features that are unique to monophonic synthesizers. There are various creative ways in which you can produce chords; for example you can produce two simultaneous pitches by skewing the pitch of oscillators 1 and 2, or assign different drum parts to different pitches and use them to play chords.