how to know the speed of an object in unity code example

Example 1: in unity i want to destroy a gameobject when it hits the edge of the screen

// this does take into account EVERY camera, including the editor view one
// but that shouldn't be a big deal once the game is built

void OnBecameInvisible() {
 	Destroy(gameObject);
}

Example 2: how do i limit the amount of prefabs in unity using c# script

CancelInvoke("Spawn");