Surround it with a loop. For example, if you want ten text boxes:
for (int i = 1; i <= 10; i++)
{
TextBox box = new TextBox();
box.Text = "aa" + i.ToString();
box.ID = i.ToString();
form1.Controls.Add(box);
}
But you should really, really be doing this within the form1 class and instance - not outside it. Let a class take care of it's own content!