Как увеличить количество выполняемых потоков?

public class Download : IManager
    {
        private Uri uri;
        private string file;
        public event EventHandler<DownloadInfoEventArgs> AlarmStartedDownload;
        public Download(Uri uri, string file)
        {
            this.uri = uri;
            while (true)
            {
                if (System.IO.File.Exists(file))
                {
                    file += "_";
                }
                else
                {
                    this.file = file;
                    break;
                }
            }
        }
        public string FileName
        {
            get { return file; }
        }

        public void StartDownload()
        {
            WebClient webClient = new WebClient();
            Thread thread = new Thread(()=> webClient.DownloadFileAsync(uri, file));   //запуск потока
            thread.Start();
            webClient.DownloadProgressChanged += (s, e) => OnAlarmDownloadStarted(e.ProgressPercentage);
        }
        public virtual void DownloadProgressChanged(object sender, DownloadProgressChangedEventArgs e)    //события, не обращайте внимания ;)
        {
            OnAlarmDownloadStarted(e.ProgressPercentage);
        }
        public virtual void OnAlarmDownloadStarted(int progress)
        {
            AlarmStartedDownload.Invoke(this, new DownloadInfoEventArgs(progress));
        } 
    }