re,
copie cette macro sur la page code de l'onglet Feuil1
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Column = 2 And Target.Value <> "" Then
Range("C" & Target.Row).Formula = "=COUNTIF($B$2:$B" & Target.Row & ",$B" & Target.Row & ")+1"
End If
End Sub