Can i set Max passcode length with this script?,Can i set a max of numbers with this script?

string PASSCODE;

void Start()
{
    //Set 'PASSCODE' to the last entered passcode
    PlayerPrefs.GetString("passcode", PASSCODE);
}

void Update()
{
    //Keep Updating the 'PASSCODE' to what the player Entered
    PlayerPrefs.SetString("passcode", PASSCODE);
    if(PASSCODE.Length > 6)
    {
        PASSCODE.Length.Equals(6);
    }
},   string PASSCODE;
void Start()
{
    //Set 'PASSCODE' to the last entered passcode
    PlayerPrefs.GetString("passcode", PASSCODE);
}

void Update()
{
    //Keep Updating the 'PASSCODE' to what the player Entered
    PlayerPrefs.SetString("passcode", PASSCODE);
    if(PASSCODE.Length > 6)
    {
        PASSCODE.Length.Equals(6);
    }
}

Like this:

string PASSCODE;
void Start()
{
    PASSCODE = PlayerPrefs.GetString("passcode", PASSCODE);
}

void Update()
{
    if(PASSCODE.Length > 6)
    {
        PASSCODE = PASSCODE.Substring(0, 6);
    }
    PlayerPrefs.SetString("passcode", PASSCODE);
}

This cuts off any additional characters after 6 characters. Note your original code did not actually load the string from playerprefs. The GetString method returns the stored string. The second argument you pass to GetString is just the default value that is returned in case the pref was not found.

Usually you would execute such code inside an OnChange callback or however / whereever you actually change the string. Executing this code every Update is pretty much a waste of performance. Though since that’s all you’ve posted, here we are ^^.