Spawning/Destroying objects inproperly

Hello
I've been trying to spawn several objects in 6 different spawnpoints. I've managed to spawn them successfully so far, but they were causing problem by overlapping each other. To prevent that I attached to all objects that I wanted to be spawned: a rigidbody (without gravity), small sphere collider (set as trigger) and made a class called 'StopOverlapping' that used OnTriggerEnter to check for collisions and respawn an object. Unfortunately It didn't work as intended - there were randomly from 0 to 6 objects spawned (mostly 0) even though I tried setting 'thingstospawn' variable to 100..
There should be 6 spawned every time.
Here's the code:

 public class GameController : MonoBehaviour {
     
     public Transform[] SpawnPoints;
     public float spawnTime = 1f;
     public GameObject[] items;
     public int items_num; 
     public static int thingstospawn;
 
     void Start()
     {
 
         Invoke ("Spawn", spawnTime);
         thingstospawn = 6;
 
     }
     void Spawn ()
     {
 
         for (int i = 0; i < thingstospawn;) {
 
 
             items_num = Random.Range (0, 5);
             int spawnPointIndex = Random.Range (0, SpawnPoints.Length);
 
             Instantiate (items [items_num], SpawnPoints [spawnPointIndex].position, SpawnPoints [spawnPointIndex].rotation);
             i++;
         }
     }
 
 }

and the 'StopOverlapping class:

 public class StopOverlapping : MonoBehaviour {
 
     
 void OnTriggerEnter(Collider other)
 
     {
         if (tag == "Item") {
             GameController.thingstospawn = GameController.thingstospawn +1;
             Destroy(gameObject);
         }
 }
 }