In the single player campaign, two sides in a battle is all that is allowed, no neutrals as far as I recall. It's very unlikely you'll face more than a couple of foes at once because they have to be allied with one another and AI allies being within the same space in RTW is pretty rare unless there's some kind of Toffee eating convention on that the AI just has to attend, which is obviously a bit of a rariety. I suppose in Italy is the greatest chance you have of facing up to numerous opponents, such as the example in the previous post as the Romans love to walk into each other's territory, typically for no real reason whatsoever.