SceneManager.loadedSceneCount

Description

The number of loaded Scenes.

Returns the current number of loaded Scenes. This doesn't include the Scenes that are currently loading or unloading. For the total number of Scenes see SceneManager.sceneCount.

using UnityEngine;
using UnityEngine.SceneManagement;
using UnityEditor;

public class Example
{
    // adds a custom menu item which displays the number of open Scenes in a Dialogue Box
    [MenuItem("SceneExample/Number Of Scenes")]
    public static void NumberOfScenes()
    {
        EditorUtility.DisplayDialog("Number of loaded Scenes:", SceneManager.loadedSceneCount.ToString(), "Ok");
    }
}