This is a question that is asked often. At the top of this page is a "Search" link. It helps to check if your question has been asked before!
Try this link:
http://www.mrexcel.com/board2/viewtopic.php?t=102878&highlight=invoice
I use this one:
Private Sub Workbook_Open()
'Place this code in the "ThisWorkbook" Module
'to increment on the opening of the workbook.
'Sub Worksheet_Activate()
'Use the "Activate" Sub in a Sheet Module
'to increment on a sheet selection.
[A3].Value = [A3] + 1
ActiveWorkbook.Save
MsgBox "Each time this sheet is opened" & Chr(13) & "the starting number is increased." _
& Chr(13) & "And this blank template is saved" & Chr(13) & _
"with the new starting number!" & Chr(13) & Chr(13) & _
"Please, save any added data" & Chr(13) & "with a new file Name!" & Chr(13) & Chr(13) & _
"To preserve system integrity" & Chr(13) & "follow these directions!"
End Sub