Space Shooter: How can I spawn the asteroids more than once?

When I run Space Shooter the asteroids spawn only once.
When I press the ‘R’ key they spawn, but only once again.
How can I make them spawn multiple times in the same run?

There is a nice function for this exactly

using UnityEngine;
using System.Collections.Generic;

// Starting in 2 seconds.
// a projectile will be launched every 0.3 seconds

public class ExampleScript : MonoBehaviour
    public Rigidbody projectile;

    void Start()
        InvokeRepeating("LaunchProjectile", 2.0f, 0.3f);

    void LaunchProjectile()
        Rigidbody instance = Instantiate(projectile);

        instance.velocity = Random.insideUnitSphere * 5;

Thank you for your response.
I added your script to the Game Controller in the Inspector and dragged the Asteroid object to the Projectile slot in the script. It didn’t work. Then, I added the script to the Main Camera instead of the Game Controller. It didn’t work. What am I doing wrong?

Instantiate(projectile, startPos.transform.position, Quaternion.identity);
You could just try and call this multiple times and it should spawn it as many times as you call it.