Reload after certain amount of shots?

Here is my (machine) gun script:

var fireRate : float = 0.1;
var Prefab: Transform;
private var nextFire = 0.0;
var speed : float = 50;
function Update () {
    if(Input.GetKey("mouse 0")&&Time.time > nextFire){
    nextFire = Time.time + fireRate;
    var copy = Instantiate(Prefab,GameObject.Find("spawnPoint").transform.position,transform.rotation);
    copy.rigidbody.velocity = transform.TransformDirection(Vector3.forward * speed);

}

}

and what I want to do is say when I reach 20 shots I have to press r to reload also a box in the corner saying how many shots I have left or how many shots I’ve taken.
Thank you

Look up GUI.Label and Input.GetButtonDown.