For me its adding 5 controls.
Check the slightly modified code below(Instead of Name I have put ID):
protected void Button1_Click(object sender, EventArgs e)
{
int j = 5;
for (int i = 1; i <= j; i++)
{
CheckBox objcheckbox = new CheckBox();
objcheckbox.ID = i.ToString();
objcheckbox.Text = i.ToString() + "checkbox";
Panel1.Controls.Add(objcheckbox);
}
}