Auto Generate NUmber

With a macro

Option Explicit

Private Sub Workbook_Open()
Sheet1.Cells(1, 1).Value = Sheet1.Cells(1, 1).Value + 1
End Sub

Copy the Excel VBA code
Select the workbook in which you want to use the code
Hold the Alt key, and press the F11 key, to open the Visual Basic Editor
In the Project Explorer, find your workbook, and open the list of Microsoft Excel Objects
Right-click on the ThisWorkbook object, and choose View Code
Where the cursor is flashing, choose Edit | Paste