VuforiaConfiguration.GenericVuforiaConfiguration a = new VuforiaAbstractConfiguration.GenericVuforiaConfiguration();

        a.MaxSimultaneousImageTargets = 3;

    if (TrackerManager.Instance.GetTracker<ObjectTracker>() != null)
        {
            TrackerManager.Instance.GetTracker<ObjectTracker>().Start();
        }
  

  if (TrackerManager.Instance.GetTracker<ObjectTracker>()!=null)
        {
            TrackerManager.Instance.GetTracker<ObjectTracker>().Stop();
        }