Collections of troops are organised by the tile they are occupying with a maximum of six units per tile. This does not mean that there can only be twelve units in a battle when two maxed-out, six-unit groups bump into each other. There can in fact be up to forty two units in a battle, as adjacent unit stacks also count toward battlefield attendance.
There are ways to increase this cap past forty two but they are pretty hard to do; there are spells like the arcane rift that summons a random unit per combat round and other spells. Charming or otherwise controlling an enemy unit does not create a new unit so it neither removes effects nor changes the number of total units in the battle.