CBaseEntityList overrides for NT from SourceMod (untested)

nt_cbaseentitylist.sp

#include <sourcemod>
#include <dhooks>

#pragma semicolon 1
#pragma newdecls required

static int _numAdds = 0;

public void OnPluginStart()
{
    Address vmt = view_as<Address>(0x22502528);
    HookVirtual(vmt, 0, OnAddEntity);
    HookVirtual(vmt, 1, OnRemoveEntity);
}

void HookVirtual(Address vmt, int index, DHookCallback cb)
{
    DynamicHook dh = new DynamicHook(index, HookType_Raw, ReturnType_Void, ThisPointer_Address);
    if (!dh)
        SetFailState("Dynamic hook failed");
    DHookAddParam(dh, HookParamType_ObjectPtr);
    DHookAddParam(dh, HookParamType_Object, 4);
    dh.HookRaw(Hook_Post, vmt, cb);
    delete dh;
}

stock MRESReturn OnAddEntity(int pThis, DHookParam hParams)
{
    ++_numAdds;
    PrintToServer("  + OnAddEntity callback: 0x%X (num adds: %d)", pThis, _numAdds);
    return MRES_Ignored;
}

stock MRESReturn OnRemoveEntity(int pThis, DHookParam hParams)
{
    --_numAdds;
    PrintToServer("  - OnRemoveEntity callback: 0x%X (num adds: %d)", pThis, _numAdds);
    return MRES_Ignored;
}