For i as Integer = 0 To nAnzahl - 1 'nAnzahl variiert zur Laufzeit
    Dim f As New MyForm
    f.TopLevel = False
    Me.Controls.Add(f)
     f.Show()
     f.BringToFront()
Next

For Each f As Control In Me.Controls
    If TypeOf (f) Is MyForm Then
        CType(f, MyForm).Close()
        CType(f, MyForm).Dispose()
         f = Nothing
     End If
Next