In Heroes of Might and Magic III, it was like this:
Few: 1-4
Several: 4-9
Group: 9-19
Horde (now called "many"): 19-50
Army: 50-99
Hundreds: 99-200
Cloud: 200-500
Darkness: 500-1k
Legion: 1k and more
This is just from my personal observations, but it seems correct.
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