How can I find how many pages printed ?

As this is a System.Windows.Forms application, normally you need to use the class System.Drawing.Printing.PrintDocument: http://msdn.microsoft.com/en-us/library/system.drawing.printing.printdocument.aspx[^].

Now, look at the small code sample at the end. It should how to print, and, in particular, how to print a page by adding a handler to the invocation list of the event System.Drawing.Printing.PrintDocument.PrintPage, see http://msdn.microsoft.com/en-us/library/system.drawing.printing.printdocument.printpage.aspx[^],
http://msdn.microsoft.com/en-us/library/system.drawing.printing.printpageeventargs.aspx[^].

Add some counter in the implementation of the handler; and you are done:

void PrintADocument(SomeData someData /* ... */) {
   // ...
   PrintDocument pd = new PrintDocument();
   uint pageCount = 0;
   pd.PrintPage += (sender, eventArgs) { // in lambda form, exact types of the parameters can be
                                         // inferred from the event type
      System.Drawing.Graphics graphics  = eventArgs.Graphics; // see the link above
      RenderThePageUsingTheInstanceOfGraphics(graphics, someData);
            // whatever you want to render on this page
      pageCount++;
   }; // pd.PrintPage handle
   pd.Print();
   // at this moment, you know how many pages are printed, read pageCount 
   // ...
} //PrintADocument

In this code, I also used an interesting feature, related to anonymous methods and the use of the stack variable pageCount, called closure. This is interesting enough to learn.
http://en.wikipedia.org/wiki/Closure_%28computer_science%29[^],
http://en.wikipedia.org/wiki/Anonymous_method[^],
http://en.wikipedia.org/wiki/Anonymous_method#C.23_lambda_expressions[^].

—SA