WCF limit for number of concurrent threads from one client

I have created a wcf service in an asp.net application that also uses a Silverlight application that only has one method:

public void TestMethod(int idRequest)
    {
                System.Diagnostics.Debug.WriteLine(idRequest);
                System.Threading.Thread.Sleep(new TimeSpan(1, 0, 0));
    }

      

        
        
        
      

    

I activate this method asynchronously from the silverlight app by clicking on the button:

 private static int countRequest = 0;

    private void Button_Click(object sender, RoutedEventArgs e)
    {
                countRequest++;

                WCFServiceManager.WCFServiceClient serviceRef = new WCFServiceManager.WCFServiceClient();
                serviceRef.TestMethodAsync(countRequest);
    }

      

        
        
        
      

    

it is possible to activate the method only 6 times (I watch VS, window output), other requests are on the line, and although one of the 6 threads will not complete its execution, a new one will not be launched in the Test method. More than 6 requests are sent from the client (it is displayed in httpFox). What is the limitation related to? How can we increase the number from 6 to 50, for example? Thank you in advance. I appreciate your help.

WS 2010, Window Output: imageshack.us/a/img580/1416/outputqn.png

HttpFox: imageshack.us/a/img546/6461/httpfox.png

Project, vs2010, silverlight 5.0