Think of the effort required under that culture to feed 20,000 people - basically they would be so busy surviving there would be nobody available for fighting.

Are you assuming they were living off the land to rationalize the group size and their activities? These people were expert at building up stores and I think there would be a lot of preserved (dried) food traveling to such an encampment. Water was the one necessity that wouldn't travel easily with such large populations. Living a nomadic life doesn't mean they didn't know where their next meal was coming from.

SRS