This unit can make any number of defensive-fire attacks each phase.
Q: Can you please explain how the Suppressive Fire special ability works?
A: Units with Suppressive Fire can Defensive Fire once each time an enemy unit moves from an adjacent hex to another adjacent hex. This means it can attack the same enemy unit twice or more if that enemy moved from an adjacent hex to another adjacent hex twice or more.
Q: So does this mean any units with this capability can defensive fire against the same enemy unit or an unlimited number of units, as many times, as the defensive fire condition presents itself?
A: The standard rule is that a unit can make one defensive fire attack per phase, period. A unit with Suppressive Fire can make as many defensive fire attacks as it wants. It doesn't matter whether it attacks five different units or the same unit five times. It can attack only once per enemy move, but it can react to every enemy move around it.