...

List<string> Inventory = new List<string>();

    void Start()
    {
        for (int i = 0; i < 8; i++)
        {
            Inventory.Add(PlayerPrefs.GetString("Inventory" + i));
            Debug.Log(PlayerPrefs.GetString("Inventory" + i));
            Debug.Log("Inventory" + i);
        }
        Debug.Log(Inventory[1]);
    }

    public void AddItem(string item)
    {
        Debug.Log(item);
        Inventory.Add(item);
        Debug.Log(Inventory[0]);
    }

    void Update()
    {
        if (Input.GetKeyDown(KeyCode.F5))
        {
            Debug.Log(Inventory[0]);
            Debug.Log("Saved");

            for (int i = 0; i < Inventory.Count; i++)
            {
                PlayerPrefs.SetString("Inventory" + i, Inventory[i]);
                Debug.Log("Inventory" + i);
                PlayerPrefs.Save();
            }
            Debug.Log(Inventory);
        }
    }