One person can take down about 8 people.
Two people can take down about 15 people.
Three people can take down up to 18 (by then people would have already run)
Four people can take down at least 25.
Each subsequent gang member improves firepower, but also increases chaos, which explains the exponential decrease in casualties. Also there's not an infinite number of human beings. By the time you've killed a bunch, the rest have fled the scene.
If you manage to put up a four man defense on, say a bank, you'd manage to kill maybe two or three police officers before being shot down/gassed.
It's perfectly possible that one member of the gang may accidentally shoot another member. Life isn't call of duty, so you don't have any indication who's friend and who's foe.