How to make randomly generated levels from a set of prefabs?

I have this snippet for you ;) (This is for 2D but you can modify it to work with 3D)

I must tell you that this is not a very good script, it have some limitations: It can only put a prefab in front of a prefab by a value you specify (this value is used for every prefab)

This thing does for some reason not seem to lag the game out when it is spawning the prefabs. I spawned over a million prefabs (it took a while XD) and the spawning did not lag the game, it only lags when there is too many objects

 using UnityEngine;
 using System.Collections;
 
 public class levelGenerator : $$anonymous$$onoBehaviour {
 
     public Transform[] prefabs;
     public int currentX = 0;
     public int NumberOfPieces = 500;
     public int currentPieces = 0;
     int prefabLength = 0;
     int rndValue;
     Random rnd = new Random();
 
     // Use this for initialization
     void Start () {
         prefabLength = prefabs.Length;
         currentX = 5;
         
     
     }
     
     // Update is called once per frame
     void Update () {
 
         if (!(currentPieces == NumberOfPieces))
         {
             currentPieces += 1;
             rndValue = Random.Range(0, prefabLength);
 
             Instantiate(prefabs[rndValue], new Vector3(currentX, 0, 0), Quaternion.identity);
             currentX += 5;
 
 
         }
 
     }
 }