How can I add ammo to my gun script?

Here it is:

 if (Input.GetButton (PlayerInput.Fire1) && _fireCounter > fireDelay) 
         {
             muzzle.GetComponent<AudioSource>().Play ();
             muzzle.GetComponent<ParticleSystem>().Play ();
             _fireCounter = 0f;
 
             RaycastHit hit;
             if (_playerController.aim) {
                 if (Physics.Raycast (_ray, out hit, 100f)) 
                 {
                     Instantiate (bulletHole, hit.point, Quaternion.FromToRotation (Vector3.up, hit.normal));
                 }
             } else 
             {
                 if (Physics.Raycast (muzzle.position, muzzle.forward, out hit, 100f)) 
                 {
                     Instantiate (bulletHole, hit.point, Quaternion.FromToRotation (Vector3.up, hit.normal));
                 }
             }
 
             StartCoroutine (MuzzleFlash ());
         }