Like Backstreet 7, Clearing 12?
That's how many gang members are part of said location, though usually no more then 5 will actually be present when you go in-person.
You can use this to your advantage to slowly whittle the gang down though. Kill the guys in the street then break off, then load back in, rinse and repeat til there are only 1-3 for a given location. That way when you have the final confrontation with the Gang Leader, you won't have as many enemies to fight. Though if the Gang Leader has another location I think they can pull members in for the final fight, so be mindful of that.
Pretty funny, cause you can get it to a point where you just fight the Gang Leader by themself in the final fight heh.
If your decent enough, fighting gang members can be a really good way to level one-handed, two-handed, and athletic skills when you start the game. Since unlike the arena XP gained isn't nerfed.
Other then some quick leveling and some good loot early on, not really much point since you'll just get really bad relations with the Gang Leaders. I guess you could do it to screw with rival Kingdoms recruitment pools - since if a Notable loses to much power they disappear. Probably not worth the effort though.
Maybe just me, but something with the game has been screwed with, it use to be real easy to summon Companions in Towns. Now I'm lucky if 1 Companion will even follow me around. Kind of puts the kabosh on going "Gangs of New York" for this sort of thing. 2 on 1 isn't too hard with Thugs, but anymore then that gets tricky and you probably want at least a companion if you're going to take on more.