[Lux]

PHP Code:

#include <sourcemod>

#pragma semicolon 1
#define PLUGIN_VERSION "1.0"
#pragma newdecls required

ConVar g_cDifficulty;

public Plugin myinfo =
{
    name = "[L4D2] Difficulty Adjuster",
    author = "Tak (Chaosxk)",
    description = "Adjusts difficulty based on number of clients in server.",
    version = PLUGIN_VERSION,
    url = "https://forums.alliedmods.net/showthread.php?t=297009"
}

public void OnPluginStart()
{
    if ((g_cDifficulty = FindConVar("z_difficulty")) == null)
    {
        LogError("[SM] Can not find convar z_difficulty");
        SetFailState("Can not find convar z_difficulty");
    }
    UpdateClientCount(GetSurvivorCount());
}

public void OnClientPostAdminCheck(int client)
{
    if (IsFakeClient(client))
        return;
    
    UpdateClientCount(GetSurvivorCount());
}

public void OnClientDisconnect(int client)
{
    if (IsFakeClient(client))
        return;
    
    UpdateClientCount(GetSurvivorCount());
}

void UpdateClientCount(int count)
{
    switch (count)
    {
        case 4:
        g_cDifficulty.SetString("easy");
        case 8:
        g_cDifficulty.SetString("normal");
        case 12:
        g_cDifficulty.SetString("hard");
        case 16:
        g_cDifficulty.SetString("impossible");
    }
}

int GetSurvivorCount()
{
    int iCount = 0;
    
    for(int iClient = 1; iClient <= MaxClients; iClient++) {
        if(!IsClientInGame(iClient) || IsFakeClient(iClient) || GetClientTeam(iClient) != 2)
            continue;
        
        iCount++;
    }
    return iCount;
} 


Maybe work? :O