Dam the Man wrote:
"If a player has a number of units in an area that exceeds thatAs per the highlighted section this doesn't apply while the combat is ongoing.
area’s unit capacity, he must destroy units of his choice in
that area until the number of units is equal to or less than the
area’s unit capacity. This is only checked after resolving an
order and at the end of each Event Phase." (RR, p.13)
The relevant rule part for this question is the 4th bullet of the combat section on page 4:
A contested area cannot contain more than five attacking units at the start of combat.The part I highlighted is important since nothing else limits you to 5 units during combat.