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 chargee 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.
For example, a unit of Night Goblins declares a charge against a unit of High Elf Archers. The Elves could stand and shoot at the Goblins, but think they might well be charged by the Wolf Riders lurking nearby so elect simply to Hold in response to the Night Goblin charge. As it turns out, the Wolf Riders do then declare a charge against the Elves, who take the opportunity to Stand and Shoot against this second charge. Finally, the Orc and Goblin player unleashes his true attack – a charge by a unit of Black Orcs. Having already 'used up' their Stand and Shoot, the Elves declare that they will turn tail and Flee!