COUNTHIDDENSHEETS

COUNTHIDDENSHEETS()
Returns the number of hidden worksheets sheets in the active workbook.

REMARKS
You can use the built-in worksheet function SHEETS() in Excel 2013 to return the number of sheets in the active workbook.

Public Function COUNTHIDDENSHEETS() As Integer 

End Function 

For instructions on how to add this function to a workbook refer to the page under Inserting Functions