Try this script:

var grenades = 20;

function Update () {
    if (Input.GetButton ("Fire1")) {
        grenades --;
        //Put your firing script here.
    }
}

function OnGUI () {
    GUI.Label (Rect (25, 25, 100, 30), "Ammo: " + grenades);
}

Whenever you fire it outputs the amount of ammo you have and updates it on the screen. Note: This does not shoot a projectile. It just keeps track of your ammo.