In the Fate series, Heroic Spirits are summoned as Servants, originated from the histories, mythologies, and fiction of many countries. This pool is for images depicting at least five unique Servants who all belong to the same nation. This doesn't include variants of the same character - they all have to be different characters with completely different histories. The first image of this pool is a good example of what to add.
The Knights of the Round Table don't apply here - they have their own tag for any groupings of them.