Once the number of NCPs gets into the double digits it doesn’t make a whole lot of sense to actually play the individual characters in combat. In this case the NPC battle is narrated with the PC party being able to influence the outcome through their success or failure in smaller battles where most of the combatants matter. When the storyteller ends up controlling more than triple the number of units the PCs do that is probably a good time to change things to a simultaneous battle and war.