EditorSceneManager.loadedSceneCount

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public static int loadedSceneCount;

Description

The number of loaded Scenes.

Returns the current number of loaded Scenes in the Editor.

using UnityEditor;
using UnityEditor.SceneManagement;

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:", EditorSceneManager.loadedSceneCount.ToString(), "Ok");
    }
}