Camera.allCameras  

var count : int = Camera.allCameras.Length;
print ("We've got " + count + " cameras");

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public int count = Camera.allCameras.Length;
    void Example() {
        print("We've got " + count + " cameras");
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public count as int = Camera.allCameras.Length

    def Example():
        print((('We\'ve got ' + count) + ' cameras'))