You have to loop recursively. Try like this.

private void Page_Load(object sender, System.EventArgs e)
{
    LoopButtons(Page.Controls);
}

private void LoopButtons(ControlCollection controlCollection)
{
    foreach (var control in controlCollection)
    {
        if (control is Button)
        {
            // This is sample code
            // You get each button here
            // Do something else here
            Response.Write(((Button)control).Text);
        }

        if (control.Controls.Count > 0)
        {
            LoopButtons(control.Controls);
        }
    }
}