Question Unity random spawn objects

Hello!
I really need help with Unity for my uni project. I have chest collectibles that spawn at random spawn points, the problem is that I need only about 15 of them to spawn, but they spawn infinitely. I don't know how to fix this. Also, maybe somebody knows how to prevent them from overlapping.
Here's the code:

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class GameController : MonoBehaviour
6. {
7.     public int numberRandomPositions = 8;
8.     public int ChestCount = 0;
9.     public GameObject chestPrefab;
10.     public List<Transform> spawnpoints = new List<Transform>();
11.     float chestTimer = 0f;
12.     float minX;
13.     float maxX;
14.     float minY;
15.     float maxY;
16.  
17.     void Start()
18.     {
19.       Vector3 viewportBottomLeft = Camera.main.ViewportToWorldPoint(new Vector3(0f, 0f, 0f));
20.       Vector3 viewportTopRight = Camera.main.ViewportToWorldPoint(new Vector3(1f, 1f, 0f));
21.    
22.  
23.  
24.         minX = viewportBottomLeft.x;
25.         maxX = viewportTopRight.x;
26.         minY = viewportBottomLeft.y;
27.         maxY = viewportTopRight.y;
28.  
29.     }
30.        
31.     // Update is called once per frame
32.     void Update()
33.     {
34.        chestTimer += 1f * Time.deltaTime;
35.         if (chestTimer >= 5f)
36.         {
37.          for (int i = 0; i < 2; i++)
38.          {
39.           int spawnIndex = Random.Range(0, spawnpoints.Count -1);
40.           Instantiate(chestPrefab, spawnpoints[spawnIndex].transform.position, transform.rotation);
41.          }
42.             chestTimer = 0f;
43.         }
44.  
45.     }
46. }