Cavalry Charges and Opportunity Fire

[J6A]

When a cavalry charge is done (or extended assault, for that matter), do the units move to the assault hex as a stack or individually.  It's a fire activation, not a move activation, and it's not sure.  Under normal circumstances, a unit can take 2 opportunity fire shots, potentially affecting 2 units.  If a unit does opportunity fire at a stack of charging units, it could affect 3 with 1 shot.  For arguments sake, let's say it disrupts all 3 charging units.  Then another stack charges and it takes its second opportunity fire shot.  Now it has potentially affected 6 units in 1 turn.  That seems high.

However, I don't see anything in the rules addressing this, so I'm not sure and figured I'd ask for input.