Option Explicit
Sub test()
Dim p As String, ws As Worksheet, x As String
p = "Lettre "
For Each ws In Worksheets
If ws.Name Like p & "*" Then
If Len(Mid(ws.Name, 8)) > Len(x) Or Mid(ws.Name, 8) > x Then x = Mid(ws.Name, 8)
End If
Next ws
If Asc(Right(x, 1)) < 90 Then
x = Mid(x, 1, (Len(x) - 1)) & Chr(Asc(Right(x, 1)) + 1)
Else
x = x & Chr(65)
End If
Sheets("modele").Copy After:=Sheets(Sheets.Count)
Sheets(Sheets.Count).Name = p & x
End Sub