You'll also want to install SuperVersus:
https://forums.alliedmods.net/showthread.php?p=830069
Put SuperVersus plugin
"/left4dead2/addons/sourcemod/plugins/..." so it reads
"/left4dead2/addons/sourcemod/plugins/l4d_superversus.cfg"
and place l4dtoolz in the same directory as you have sourcemod and metamod:
"/left4dead2/addons/..." so it reads
"/left4dead2/addons/l4dtoolz"
edit your server.cfg located at "/left4dead2/cfg/server.cfg" add these cvars after installing SuperVersus:
so it includes (if you wanted 6 players on each team):
open it up in a text document like notepad, etc
sv_visiblemaxplayers 12
sv_maxplayers "12"
sv_removehumanlimit "1"
sm_cvar z_max_player_zombies 6
l4d_survivor_limit 6
l4d_infected_limit 6