Thirteen is, to me, the second most likely number, and it could very well be correct.
What do you believe is the most likely number? 3? I actually think it makes a lot of sense, and more easy to manage (from a writing stand point). Also it would be so dang difficult if you needed to banish all 3 to get Nemesis out of reality.
As for roles, I think we've seen what each is good at, and their limitations.
Before is leading the charge (from the Fore-front 
) and willing to fight, but not all that great at it. He can debuff, and probably buff his own team, but he wasn't all that impressive in the bar fight until he whammied.
He referred to himself as "the harbinger". The meaning is: a person or thing that announces or signals the approach of another... So I think his main, effective weapon is his whammy. His psychic attack was even able to neutralize the Erlking and his crew. What makes this so dangerous is he could cripple a forces front lines, leaving them helpless to attackers. His failures at Mac's and during the battle at the Island is that his forces were limited (he didn't have any at Mac's).... Now imagine if instead he'd have been leading the charge is Battle Ground... All those people would have dropped their shotguns, paralyzed as they got ripped to shreds by the Fomor invading force. Or imagine that when Harry and Eb faced off against those Corner Hounds if he'd used that. Eb would have been defenseless and ripped apart in seconds instead of using this shields long enough for Harry to do this thing. So Before is actually a terrifying entity. He just has a very specific role, and he didn't have the support he needed.
Beside is the spymaster who can manipulate the outcome of battles from the Side-lines (I crack myself up ) but he's even worse as a combatant.
He's struggled to control his hosts efficiently multiple times (Lea defied him, and he couldn't use Sith to his fullest combat capabilities).
LoL... Beside is a really odd, but frightening enemy... By odd I mean I'm not sure if it's Jim's well thought out design (I hope), or if it's plot armor. What I mean is that on one hand Beside seems really frightening. Can possess people, master manipulator, Angels can't even detect him, and yet the targets he chooses don't seem that great. Why stop with Aurora, Leah, and Maeve? Why not Mab or Titania? Sure it might take longer but once Nemfected the hosts don't seem to have much way of defending themselves from what I can tell.. But maybe they are too powerful. Ok that makes sense. Why Justine? Why not Thomas? He's closer to Harry, he's more physically capable than Justine. Instead of blackmailing him into attacking, why not just possess him and do it? Why not possess Eb or Langtry? Surly possessing Langtry would be a great thing for the Outsiders, and yet it doesn't seem to be the case from what I can tell... So Nemesis targets are a bit strange to me. I'm thinking his weakness is simply a strong will. He says "I am the flaw the corrupts", "the infected wound"... It may be that the victim has some sort of emotional weakness that's easier to exploit. I guess you could say that Beside is a bit of a coward. It prey's on the vulnerable but doesn't do well against someone who has a stronger will. I also thought that Jim said something about older wizards being more set in their ways and that's why they aren't Nemfected but that was a long time ago and I might not be remembering correctly.
Behind is the most potent, as stated in the books, because he's not stuck beHind (okay that one wasn't as good) some weak or resistant host. He's the assassin in the group that gets physical, but also got banished by a (starborn) kid because he underestimated his enemy.
Behind I think is the one who uses pure brute strength to kill. He doesn't hide away, or lead the charge. He is the weapon. He doesn't need backup, and once marked, he will always be following you, waiting.
So, in summary, there's the Cult Leader (Before), the Mastermind (Beside), and the Soldier/Assassin (Behind).
For me I see Before as the battering ram. He stuns the front line, allowing his forces to rip them to pieces. Beside creates chaos behind enemy lines. Sows distrust, creates disasters, and is always trying to exploit weaknesses. Beside is the saboteur I think. Behind is the perfect assassin.. Works alone, extremely dangerous, kills without remorse...
To be honest they are dangerous but after seeing what Mab can do, they don't feel as big as her. That being said we've always seen from Harry's point of view, and he's a Starborn. If he wasn't on the scene, the Walkers may have not really had any opposition strong enough to do anything.
