Percom controller for the Atari computer runs FOUR drives.

Here the Percom AT88-SPD controls four SA400-compatible mechanisms:
A single-sided 40 track 5 1/4" mech (90k)
A double-sided 80-track 5 1/4" mech (720k),
A double-sided 40-track 5 1/4" mech (360k),
and a double-sided 80-track 3 1/2" (720k) mechanism.