// START DETECT
private FaceDetection _faceDetection = new FaceDetection();
_faceDetection.OnDetectFace += new FaceDetection.FaceDetectedHandler(GetFacePosition);
_faceDetection.StartDetect();

private void GetFacePosition(object sender, FaceDetectEventArgs e)
{
    // number of people
    var people = e.ResultFrame.Count
}

// STOP DETECT
_faceDetection.StopDetect();