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