A unit might well be called upon to make several charge reactions over the course of a Charge sub-phase, if it is charged by several enemy units.
A unit can only Stand and Shoot once in a Charge sub-phase – there is no time to reload. This is not to say that the Stand and Shoot must be used against the first charging unit – the charged unit can opt to Hold against the first charge and Stand and Shoot against the second charge. It could even then elect to Flee in reaction to a third charge if it wished! A unit that Flees once in a Charge sub-phase will have to keep fleeing if it has more charges declared against it, as described earlier.
Once a unit has declared a Flee charge reaction, or if it is already fleeing at the start of the movement phase, it must declare and resolve a Flee charge reaction for every subsequent charge declared against it that turn. Note that a unit may only make an actual Flee move once per phase.