Private Function GetNodeCount(ByVal nc As TreeNodeCollection) As Integer
        Dim i As Integer = nc.Count
        For Each nd As TreeNode In nc
            i += GetNodeCount(nd.Nodes)
        Next
        Return i
    End Function

Debug.WriteLine(Me.GetNodeCount(Me.TreeView1.Nodes))
'// oder:
Debug.WriteLine(Me.GetNodeCount(Me.TreeView1.Nodes(0).Nodes))