c# enemy's not spawning

Only the first 2 enemy's will spawn but none after that, also if spawnRate is higher the 0 then they don't spawn at all?

 using UnityEngine;
 using System.Collections;
  
 public class enemySpawn : MonoBehaviour {
 
    public GameObject enemy;
    private float timePassed = 0f, spawnRate = 0f, spawnTimer = 0f;
    private int numberOfEnemy = 2;
  
    void Start()
    {
       StartCoroutine(Speed());
       StartCoroutine(Spawn());
    }
  
    IEnumerator Speed()
    {
       timePassed += Time.deltaTime;
       if(timePassed >= 10.1f) {
          timePassed = 0.0f;
          numberOfEnemy++;
          spawnRate -= 0.5f;
          yield return spawnRate;
          yield return numberOfEnemy;
          yield return timePassed;
       }
    }
  
    IEnumerator Spawn()
    {
       spawnTimer += Time.deltaTime;
       if(spawnTimer > spawnRate) {
          for (int i = 0; i < numberOfEnemy; i++) {
             Instantiate(this.enemy);
          }
       spawnTimer = 0.0f;
       yield return spawnTimer;
       }
    }
 }