Uncontrollable platform spawning.

I made simple spawner like this:

 void Update() {
 
     if (PlatformPosition.spawnNewPlatform) {
         NewPlatform();
     }
 }
 
 public void NewPlatform() {
 
     if (active) {
         newTransform = transform;
         Instantiate(prefabs[Random.Range(0, prefabs.Length)], newTransform.position, Quaternion.identity);
     }
 }

And so it looks like my platform script:

 public static bool spawnNewPlatform = true;
 
 void Awake() {
     spriteSize = GetComponent<SpriteRenderer>();
     offscreenX = (Screen.width / PixelPerfect.pixelsToUnit) / 2 + offset;
 }
 
 void Update () {
 
     pos = transform.position;
     posX = pos.x;
 
     if (posX <= -offscreenX) {
 
         spawnNewPlatform = true;
     }
     else {
         spawnNewPlatform = false;
     }
 }

Now when I start the "game" the spawner create unexpected number of platforms. Sometimes it's 2 clones sometimes it's 10... I don't understand what is the problem and how to fix it. Is it frame rate issue?