Creating and Destroying GameObjects

Some games keep a constant number of objects in the sceneA Scene contains the environments and menus of your game. Think of each unique Scene file as a unique level. In each Scene, you place your environments, obstacles, and decorations, essentially designing and building your game in pieces. More info
See in Glossary, but it is very common for characters, treasures and other object to be created and removed during gameplay. In Unity, a GameObjectThe fundamental object in Unity scenes, which can represent characters, props, scenery, cameras, waypoints, and more. A GameObject’s functionality is defined by the Components attached to it. More info
See in Glossary can be created using the Instantiate function which makes a new copy of an existing object:

public GameObject enemy;

void Start() {
    for (int i = 0; i < 5; i++) {
        Instantiate(enemy);
    }
}

Note that the object from which the copy is made doesn’t have to be present in the scene. It is more common to use a prefabAn asset type that allows you to store a GameObject complete with components and properties. The prefab acts as a template from which you can create new object instances in the scene. More info
See in Glossary dragged to a public variable from the Project panel in the editor. Also, instantiating a GameObject will copy all the Components present on the original.

There is also a Destroy function that will destroy an object after the frame update has finished or optionally after a short time delay:

void OnCollisionEnter(Collision otherObj) {
    if (otherObj.gameObject.tag == "Missile") {
        Destroy(gameObject,.5f);
    }
}

Note that the Destroy function can destroy individual components without affecting the GameObject itself. A common mistake is to write something like:

 Destroy(this);

…which will actually just destroy the script component that calls it rather than destroying the GameObject the script is attached to.