Try this. Create a text field on your report where the control source is =Pages. You can make this invisible if you don't want it to show up on the report itself.
Then create an Event Procedure for the onPrint Event in the report header that goes something like this:
Dim rptPgs As Integer
rptPgs = Reports!yourReportName.Pages
MsgBox "This report contains " & rptPgs & " pages.