To have extra slots (make sure you have it in your host), you can use
left4downton2 or
l4dtoolz +
Gamedata.
Use these settings in your
server.cfg
If you use Left 4 downtown 2:
l4d_maxplayers 6
If l4dtoolz, use this:
sv_maxplayers "6"
sv_removehumanlimit "1"
sv_force_unreserved "1"
After all:
sv_visiblemaxplayers "6"
If you are trying to setup a Coop server, use
Multislots or
Bebop .
If you are trying to setup a Versus server, use
Superversus.
This plugin will create in your cfg/sourcemod a new file called l4d_superversus.cfg.
Edit this file and use this:
l4d_survivor_limit "3"
l4d_infected_limit "3"
Thatīs all.