Application.levelCount  

// Loads a random level

Application.LoadLevel (Random.Range(0, Application.levelCount-1));

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
void Awake() {
Application.LoadLevel(Random.Range(0, Application.levelCount - 1));
}
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

def Awake():
Application.LoadLevel(Random.Range(0, (Application.levelCount - 1)))