The dead horse of multiple attackers keeps being beaten. And I think it should. It's an important question. But does any training that anyone know of address the idea of multiple defenders? Meaning, if you're out with a group, and you're attacked, how to move to maximize the effectiveness of the group, rather than just resorting to melee combat? Think of the Roman Legions and how much more effective they were man for man because of their group oriented tactics, than their enemies who often were individual "heroic" warriors. The legions mowed them down.
I know it is unlikely that you would be traveling in a group where several people are trained in the same martial art. But it's an interesting idea nonetheless. Systema is the only art I can think of that might have this training, and I don't even know for sure about this.
I have found it takes expereince with those who have been there.
For example, I know that I have walked around in a crowd while two faced off, and told stories about one of the guys, and how he beat a police officer up and is still not in jail. *** Fiction is great *** other times you just wait in the crowd for the other guys friends to make his move then you check him and tell him he had better not as I am not alone in the crowd and I have him covered. You see these are tactics to use friends to help.
Another case is have one person face off and the second one make the first shot for a sucker punch, as the two faced off usualy are not paying attention. In my case I slapped the second one across the face, because I used my whole vision and not tunnel vision. This allowed me to see his fist ball up and for me to react and then my hand was up after the slap and ready for the 7' guy I was staring up too.
Large numbers vesus large numbers. i.e. 13 attackers and 5 defenders
Three cars pull in and empty out while we were playing hackey sack. ** Hackey Sack is not a crime

** Given position I had to take point. The other four with me were my friends and all blooded in some form on conflict as I had been. I knew I could trust them. The off duty officer retrieved his firearm still concealed, antoher guy reaches in and grabs a knife in his car, the third reaches in and grabs a tire iron. The fourth cannot obtain anything given position, and is watching and prepared, while I look down and see a tuna fish can in the parking lot. I grabbed the unopened can.
Their best fighter gets right into my chest (* much shorter than me *) and starts breathign harder to pump himself up. The talker in the back says something about us causing the problem and why did we do it. A person with me says hey man, just back as one of us an off duty officer. Their reply is that they eat officers for breakfast. I look at the talker and tell him I going to kill him. The fighter in my chest breathing says the fight is with him. I state that he is already down and the guy behind him on the right has a broken knee and the guy on the left has lost his eye all in my path to get to the guy in the back that I was telling them I was going to go after.
The fighter moved to create some space, I knew what was going on, so I hit him upside the head with the can palmed in hand. Down he goes. The guy behind him on the right had moved a little as he was afraid I was going to break his knee, so when I shoved the guy after the shot, they all fell over just like the "Keystone Cops". I then stepped on a couple and ran to the front of the building as they tried to get up. I opened the front door with out turning off the alarm, so it went off. They decided that it was best to leave at that point.
I was chastised by my friends for going past them, and they were afraid that I could have had 8 on me while the rest all had one. Which is true but I took advantage of the situation and reacted.
If you have a plan it helps. If you practice it helps. Just remember that no plan ever survived intact with contact with the enemy. So one must be willing to adjust.