Scrolling level Instantiate after cycle

When I run my level in generates 10 obstacles, similar to Mario pipes. these then scroll across the level and finish about position.x 9000, I need my script to generate some more at that point and destroy the previous ones, to make the level infinite .

at the minute obstacle is a prefab and X is the amount of pipes to spawn at start.

 using UnityEngine;
 using System.Collections;
 
 
 public class levelgeneration : MonoBehaviour {
     
     public GameObject obstacle;
     public float x = 1;
     public float moveSpeed = 0.03f;
     
     void Update () {
         transform.position += new Vector3(5, 0, 0);
         float y = Random.Range(-27.1157f, -18.05732f);
         if(x < 15) {
             Instantiate(obstacle, new Vector3(x * 12.0f, y, 0),Quaternion.identity);
             x++;
         }
 
         if (transform.position.x > 9000) {
 
 
 
                 }
 
     }
 }