Procedural Generation generating infinitely?

Unfortunately, I've written some code and have run into some errors executing it without crashing Unity. I've placed some code that should place cubes above the original cube Number times. However, when I run the code, it starts generating A TON of cubes and Unity freezes. Here is my code:

 using UnityEngine;
 using System.Collections;
 
 public class GenerationController : MonoBehaviour {
 
     public GameObject genCube;
     public Quaternion rot;
     public int J;
     public int Number;
 
     // Use this for initialization
     void Start () {
     
         float a = 0;
         while(a<Number)
         {
             Vector3 positionOver = new Vector3 (genCube.transform.position.x, genCube.transform.position.y + J, genCube.transform.position.z);
             Instantiate(genCube, positionOver , rot);
             a++;
         }
 
 
     }
     
     // Update is called once per frame
     void Update () {
 
 
     }
 }

If you can provide any, please help.