CounterScript

using System.Collections;
using UnityEngine;
using UnityEngine.UI;

public class CounterScript : MonoBehaviour
{
    [SerializeField] private Text counterText;
    private float counter = 0f;
    private bool isCounting = false;
    private Coroutine countCoroutine;

    void Update()
    {
        if (Input.GetMouseButtonDown(0))
        {
            isCounting = !isCounting;

            if (isCounting)
            {
                if (countCoroutine != null)
                    StopCoroutine(countCoroutine);

                countCoroutine = StartCoroutine(CountCoroutine());
            }

            else
            {
                if (countCoroutine != null)
                {
                    StopCoroutine(countCoroutine);
                }
            }
        }
    }

    IEnumerator CountCoroutine()
    {
        while (isCounting)
        {
            counter++;
            counterText.text = counter.ToString();
            yield return new WaitForSeconds(0.5f);
        }
    }
}