I believe the myth 2 manual has a definition for "The Legion" as being the army that swept across the west and east and defeated Balor. So in TFL, "The Legion" was just the name of one particular army, which began 20 000 strong and eventually was reduced to a mere handful of men after the final battle with balor. I think it's safe to say that during the Great War, there were many armies of varying sizes commanded by Avatara or professional generals. While this is complete speculation, I'd say all of them were much smaller than "The Legion", given that most of the time defeat seemed inevitable until the legion arrived, suggesting that it was the most massive force the west could raise. The remaining garrisons in the North and the field armies fighting at the cloudspine were probably only a few thousand men strong.
As for the number and respective sizes of legions in the time of Myth II, I don't think any information is available. Anything we say is just a guess. But there is SOMETHING to go on. Early in myth II the narrator considers a thousand thrall to be a daunting force. Yet when he mentions the Seventh Legion travelling to Scales to fight Soulblighter, it is said as though that force is sufficient in size to crush him. So we can determine either that, a) a legion was just an army that could be any size, or; b) a legion was a full army comperable to the forces in TFL, but not all of the military forces were large enough to be legions. The initial force the narrator travels with seems to be more of a police force, maybe just 200 or 300 men.
