this plugin add this cvar to l4d and l4d2
sv_maxplayers - max human players that can join your server. -1 to disable and use standart check (range 0 to 32)
sv_removehumanlimit - remove kick for players when they are more than 8 for vs and 4 for coop(0 to disable, 1 to enable)
sv_force_unreserved - if you set it to 1 before connection from lobby your server will stay unreserved and allow players to connect using connect command, this command sets sv_allow_lobby_connect_only 0, so you need to create lobby using ms_force_dedicated_server and then connect to server, your friends may see wrong max players in friend lobby, so give them ip.
1.0.0.9f for L4D1: https://forums.alliedmods.net/showpo...postcount=1123
1.0.0.9h for L4D2: http://forums.alliedmods.net/showpos...postcount=1211
1.0.0.9f source: https://github.com/ivailosp/l4dtoolz/