A battle will be able to accommodate more then two participants.
Battles begin with one entity (usually an army) attacking another entity, like another army, a NPC, a castle etc.
The battle will be visually apparent on the main map and other players, or armies from the same player, may choose to join the battle on either side.
This means, multiple armies attacking the same castle, multiple armies attacking the same NPC, lords under attack calling for help and hoping they receive it in time etc.
Keep in mind, there will always be two sides!