I mean any campaign ( Both Grand Campaign and the rest of the specific campaigns )
By enough garrison troops I mean enough in numbers so that you stand some chance to repel an enemy army's attack that comes with a full stack.
EDIT:
We can also take into account the garrison troops quality and not just numbers
Generally it is very hard to repel an attack that comes with a full stack specially if the settlement is just level 1 or 2. But do some factions have decent garrisons even in level 1 or level 2 settlements?