You've only created one button which you're changing the ID of and adding multiple times. You need to create a new button for each iteration of the loop.
for (int i = 1; i <= Convert.ToInt32(hdnCountData.Value); i++)
{
HtmlButton btns = new HtmlButton();
string id = "btn" + i;
btns.ID = id;
btns.InnerText = i.ToString();
btns.Attributes.Add("class", "circularButton");
btns.Attributes.Add("type", "button");
divBodyButtons.Controls.Add(btns);
}