Picking up an item and spawning a new one.

I'm currently trying to create a slightly more advanced version of the Unity 'Roll A Ball' tutorial. Specifically, I am trying to create a feature in which the 'Pick Up' objects will spawn around my playable area one at a time, to a maximum of 5 objects in the game at one time. With my current code, I am able to have the objects spawn in random areas and indeed to a maximum of 5, but once the game has spawned 5 objects it won't spawn anymore, despite some of the objects having been collected or deleted (Meaning there would be less than 5 in the game at one time). I'm fairly new to C# and so I wouldn't be surprised if my code is a bit rubbish :P. Thanks to anyone that can help!

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class SpawnPickup : MonoBehaviour {
 
     public int maxNumObjects = 5;
     public GameObject objectToSpawn;
     public float rangeX_Negative = -5f;
     public float rangeX_Positive = 5f;
     public float rangeZ_Negative = -5f;
     public float rangeZ_Positive = 5f;
     public float respawnTimer = 5f;
 
 
 
     // Use this for initialization
     void Start()
     {
         StartCoroutine(SpawnTimer());
     }
 
     IEnumerator SpawnTimer()
     {
 
         for (int i = 0; i < maxNumObjects; i++)
         {
             Spawn();
             yield return new WaitForSeconds(respawnTimer);
         }
     }
 
 
     void Spawn()
     {
         float randomX = Random.Range(rangeX_Negative, rangeX_Positive);
         float randomZ = Random.Range(rangeZ_Negative, rangeZ_Positive);
         Instantiate(objectToSpawn, new Vector3(randomX, 0.75f, randomZ), Quaternion.identity);
     }
 }

I also tried to add the following code in place of the SpawnTimer function code, but it still didn't work. (Not sure I got this code right though)

         if(maxNumObjects < 0)
         {
             Spawn();
             yield return new WaitForSeconds(respawnTimer);
         }