How to do if something happened make number - 1

Question

I am trying to do when i destroy all boxes something happen. My code is;

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.SceneManagement;


public class destroy : MonoBehaviour
{

private string BALL_TAG = "ball";
public AudioClip coin;
public AudioSource src;
public float numBox = 120f;
public bool isDestroyed;




private void OnCollisionEnter2D(Collision2D collision)
{
    if (collision.gameObject.CompareTag(BALL_TAG))
    {
        src.clip = coin;
        src.Play();
        Destroy(gameObject);
        isDestroyed = true;

    }
}

private void Update()
{
    boxes();
}

public void boxes()
{
    if(isDestroyed == true)
        numBox -= 1f;

    if(numBox == 119)
        SceneManager.LoadScene("mainManu");
}

private IEnumerator Two()
{
    yield return new WaitForSeconds(1f);
    Destroy(gameObject);
}

}

But it doesn't work. It is suppose to do when I broke 1 box it sends me to menu. I think its problem in "numBox -= 1f;" because I don't know hot to make this.