Surrounding, Flanking, and Backstabbing

 

I had a question from one of my players about a possibly situation that may arise in our game tonight.

We had to stop last game in the middle of a fight with a powerful trog chieftain.  Many of the chieftain's mooks are dead, leaving on the chieftain, but he's about to get surrounded.

So my question is this: what would you allow when someone is surrounded? When someone runs away they show their back, and that denies them their shield bonus and lets thieves backstab them, but surely they can't show their shield to everyone who is attacking them (or can they?).  would you allow someone to get the backstab opportunity, and if so how many and how would you determine who?

As a rule of thumb, I allow up to 8 combatants to attack one surrounded target. For every 3 combatants, a 4th combatant can attack from the rear.

 

clever rule of thumb, and easily less fiddly than some of the other options i was considering.

We've pissed off a thieves' guild, so I've been considering this a bit, and, what two allies fighting back-to-back might mean.

 X X X X
X  Y Y  X
 X X X X

So, that might imply 10 opponents surrounding, and probably 5-7 opponents able to strike a single character - or one backstab per split evenly.

Three allies:

  X X X X
 X  Y Y  X
  X  Y  X
   X X X

which kinda looks like 11-12 opponents surrounding, with 3-4 opponents able to hit a character - 1 backstab per, split evenly.

4 allies:

 X X X X
X  Y Y  X
X  Y Y  X
 X X X X

12 opponents, 3-4 opponents striking each character, 1 backstab per 2 characters.

Kinda feels like once you get to 4 or more characters fighting back to back you start reaching the magic number where less than half or none of the characters may earn a backstab on any given round.

Six:
 

  X X X X
 X  Y Y  X
X  Y   Y  X
 X  Y Y  X
  X X X X

Gets closer, at least. That's 14 on 6, so only 3 characters get backstabbed. Maybe a formation of 8 can do it.