Incrémenter

Sub test()
If [B3] <> "" Then [C3] = "N°1"
End Sub

Private Sub CommandButton1_Click()
Dim n%
n = 1
For lig = 3 To [B65536].End(xlUp).Row 'boucle 3 à derniere ligne
If Cells(lig, "B") <> "" Then Cells(lig, "C") = "N°" & n: n = n + 1
Next
End Sub

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
Dim n%
n = 1
For lig = 3 To [B65536].End(xlUp).Row 'boucle 3 à derniere ligne
If Cells(lig, "B") <> "" Then Cells(lig, "C") = "N°" & n: n = n + 1
Next
End Sub