In reading about Alfred the Great I was a bit surprised about the number of armies that were about. The Viking Great Army was a muster of the reatiners etc of several Viking lords, and the Saxon forces were similar, the retainers of a number of great nobles, the Kings forces and the fyrd. And this is to get forces of about 4000 on each side, rather than any great numbers. So of battles of this small size can contain up to 10 'armies' I'd imagine that throughout the world 6000 isn't impossible.