Was thinking through the max size of entities fighting for you in combat possible and came up with this
Iist:

You
Your primary companion
Dog
Horse
Quest NPC accompanying you
Two Conjured creatures you create in battle
Two Conjured creatures your companion creates
Two Conjured creatures your accompanying quest NPC creates

That is eleven potentially! Most I've been able to create was eight as my companion only raised one creature (not found a conjured quest NPC yet). Not tried to raise undead while my Atronachs are up.

Anyone seen more then eight?