I agree with Wyrm. Also while more than 4 kings can spawn there cant be more than 4 at one time. The extra ones are simply the ones you already killed respawning. Kaathe and Frampt are the 2 main serpents. Kaathe leeads the 8 who still serve the dark and Frampt believes he is the primordial serpent because he is the only serpent supporting the fire.
In the dark lord ending the serpents talk about how they will serve their master again. Originally all 10 served the dark lord but then frampt turned.
EDIT: They look he same because they were all mutated by the abyss into the figue that you see when you fight them. Mutate may not be the right word but it gets the idea across.
They share a health bar. It isn't even set up to hit 0 when you would have done enough damage to kill all 4 kings. This implies that they share a life source, ergo one being. If the additional ones were simply the dead ones respawning, the fight would probably have the kings being weaker and you have to make them all die within a certain amount of time from the first one. This kind of tactic is not unheard of in video games, and appears often in WoW (Granted, it's typically for groups of like 10-40 people, but still). There is no reason to think that if they could rise from the dead that quickly that you would be able to actually defeat them. It just makes no sense to me.
(I don't want to come across as stubborn. I am not clinging to this idea, I just don't find the evidence convincing enough.)
I guess you could claim that the darkness would bring everything to be the same, since it is trying to extinguish the flame of disparity, however darkness is explicitly stated to be "generated" by the flame.
For that matter, has the narrator of the opening cinematic been determined/discussed at all? Seems like something that COULD possibly be of importance to lore.