Maybe something like this
Option Explicit
'SaveAsUI Required Boolean True if the Save As dialog box is displayed due to changes made that need to be saved in the workbook.
'Cancel Required Boolean False when the event occurs. If the event procedure sets this argument to True, the workbook isn't saved when the procedure is finished.
Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As Boolean)
Dim P As String, N As String, F As String, E As String
Dim c As Long
Dim i As Long
SaveAsUI = True
Cancel = False
i = InStrRev(ThisWorkbook.Name, ".")
N = Left(ThisWorkbook.Name, i - 1)
E = Right(ThisWorkbook.Name, Len(ThisWorkbook.Name) - i)
If Right(N, 2) Like "##" Then
c = Right(N, 2)
N = Left(N, Len(N) - 2)
Else
c = 0
End If
F = Dir(ThisWorkbook.Path & Application.PathSeparator & N & Format(c, "00") & "." & E)
Do While Len(F) <> 0
c = c + 1
F = Dir(ThisWorkbook.Path & Application.PathSeparator & N & Format(c, "00") & "." & E)
Loop
Application.EnableEvents = False
ThisWorkbook.SaveAs ThisWorkbook.Path & Application.PathSeparator & N & Format(c, "00") & "." & E
Application.EnableEvents = True
ThisWorkbook.Close
End Sub