From The Wargamer, Vol.1 38

Hitler turns against Russia

RULES ANSWERS

Rule 5.2.8. (Stacking)
The stacking limit is four friendly units per hex. If a hex contains four
friendly units, an enemy unit may still enter the hex for combat.

Placement of Reinforcements.
If reinforcements are designated to appear in a specific hex, and that hex
is occupied by an enemy unit, the reinforcements may appear in any friendly
supplied hex chosen by the player receiving the reinforcements.