How to slow down my spawn rate?

I am working on a part of a script that will run a countdown timer, when the timer reaches zero it will call a function that spawns a set amount of enemies (in this case 10)

 function FixedUpdate()
 {
 timer -= Time.deltaTime; // Count down the time in seconds on timer
     if(timer <= 0)
     {
         timer = 0;
         Debug.Log("New wave started");
         StartNewWave(); //start the new wave!
     }
 }
 
 //start the new wave
 function StartNewWave()
 {
     //Set the GUI
     UpdateGUI();
     
     while (enemyCount <=9)
     {
     //spawn the enemies
         SpawnNewEnemy();
     }
 }    

That all works but my issue is that they spawn incredibly close together. The update function is what is determining their spawn rate. I want them to be more spaced out, like if I could call fixed update every half a second. The results are similar when using regular update but is looks sloppier and either way it isn't doing what I want.

I tried putting a yield WaitForSeconds (0.5); right above my SpawnNewEnemy(); call but when I did that I ended up spawning WAY too many enemies. I'm not entirely sure why.

Can anyone think of a solution to this?