A unit may not retreat onto, or through, a hex already containing
another friendly unit or an enemy unit or enemy leader.
So a single LDR blocks the retreat path of an enemy unit and the enemy units lost blocks, for each hex they can´t retreat.
Leaders Stop Retreats: A unit without an attached leader may
retreat onto a hex that contains an unattached friendly leader
(i.e. a leader alone in a hex). The leader is immediately attached
to that unit and the unit’s retreat stops in the leader’s hex. The
retreating unit will ignore any additional retreat movement.
If the single LDR is a friendly one, the friendly unit stops their retreat on the LDR hex.