How can I destroy many instantiated objects on endless game

Here is my script that I used:

 public GameObject StructureToSpawn;    
 
     GameObject spawnedStructure;
     GameObject prevStructure;
    private Vector3 StartPositionForStructures;
 
   void Start()
     {
       StartPositionForStructures = new Vector3(-30f, 0f, 0f); 
       //Start postion should be outside screen.
        SpawnStructure();
     }
 
 //The Spawn Part
     void SpawnStructure() //AtStart
     {
         spawnedStructure = GameObject.Instantiate(StructureToSpawn, 
        StartPositionForStructures, startRotation) as GameObject;
         prevStructure = spawnedStructure;
 
     }
 //The Destroy and Respawn Part
 
      void InstanciateStructure() //On update
     {
 
         if (spawnedStructure.transform.position.x < -35f) //When struct outside screen
         {
             prevStructure = spawnedStructure;
     spawnedStructure = GameObject.Instantiate(StructureToSpawn, StartPositionForStructures, startRotation) as GameObject;
             Destroy(prevStructure);
 //Or pool multiple objects into an array and destroy all...
 
         }
 
     }   
 
 void Update()
 {
  InstanciateStructure();
 spawnedStructure.transform.Translate(Vector3.left * 15f * Time.smoothDeltaTime); 
 }