Alors on va modifier un peu le code :
Private Sub Worksheet_BeforeDoubleClick(ByVal Target As Range, Cancel As Boolean)
Dim I As Byte, n As Byte
If Target.Count > 1 Then Exit Sub
If Not Application.Intersect(Target, Range("A:A")) Is Nothing Then
n = InputBox("Nombre de cellule à recopier")
For I = 1 To n
Target.Offset(I, 0) = Target + I * 7
Next I
Target.Offset(n, 0).Interior.ColorIndex = 4
End If
End Sub