Admittedly, I don't have the skills to complete a single Left 4 Dead campaign on "Expert" difficulty. I'm a little to casual with the auto shotgun. But having 12 Survivors on my team? Well...
This little Left 4 Dead modification, one that takes advantage of 12 player slots for up to a dozen survivors versus the world's infected, isn't some simple switch one needs to flip via console commands. It requires a pair of installed mods, some setting finagling, a little misdirection plus some jumping through hoops that will likely take a decent time investment to get working.
But look at all those green silhouettes! If you watch the video, stick around for the dance party at the end.
Left 4 Dead? More Like Left 12 Dead [L4Dmods - thanks, David!]