How to Retrieve Total Page number from the Sub Report

Go to Report > Report Properties > Code

Public Shared SubReportPageCount As Integer

Public Function GetSubReportPageCount() As Integer
    Return SubReportPageCount
End Function

Public Sub SetSubReportPageCount(ByVal pageCount As Integer)
    SubReportPageCount = pageCount
End Sub

In your subreport, create a text box and set its value to the following expression

=Code.SetSubReportPageCount(Globals!OverallTotalPages)

In the parent report, create a text box and set its value to the following expression

=Code.GetSubReportPageCount()

or you could use a parameter, "SubReportTotalPages" in the subreport with a value of =Globals!OverallTotalPages and pass the parameter to the parent report by adding the same named parameter to the parent report and set the value to the subreport's parameter. 

In the parent report you could then just reference the parameter

=Parameters!SubReportTotalPages.Value