Array of different rigidbody

Hy,I am new to unity and i making a simple game in which i want different balloons will fly from random position and user will destroy them when touched. One object is flying and doing as i want but i don't know how to take random objects and then make them fly...`using System.Collections; using System.Collections.Generic; using UnityEngine;

public class BallonSpawn : MonoBehaviour {

 public GameObject Ballon;
 public Rigidbody2D rb;
 public float maxpos = 4.0f;
 public float delaytimer = 1f;
 public float timer;
 public float speed = 3f;
 public int score = 0;


 // Use this for initialization

 void Start ()
 {
     timer = delaytimer;
 }

 // Update is called once per frame
 void Update ()
 {
     Vector3 movement = transform.up;
     Vector3 direction = movement.normalized;
     Vector3 velocity = direction * speed;
     rb.position = velocity * Time.deltaTime;

     timer -= Time.deltaTime;
     if (timer <= 0)
     {
         Vector3 Ballonpos = new Vector3 (Random.Range(-4.0f,4.0f) , transform.position.y , transform.position.z);
         Instantiate (Ballon, Ballonpos, transform.rotation);
         timer = delaytimer;
     }

     if (Input.touchCount > 0)
     {
         print ("ther is a touch");
     }
 }

 /*void (Collision col)
 {
     //if(other.gameObject.CompareTag("Coin"))
     if(Input.touchCount > 0)
     {
         col.gameObject.SetActive(false);
         score = score + 1;
     }
 }*/

}`