Take a hidden field on the page which will store the current tab index of the page and set the value of that hidden field to 0 by default.
Then try this is your code:
protected void btn_click(object sender, EventArgs e)
{
hdfTabIndex.Value = (Convert.ToInt32(hdfTabIndex.Value)+1).ToString();
ctlTabContainer.ActiveTabIndex = Convert.ToInt32(hdfTabIndex.Value)+1;
btnNext.ForeColor = Color.Blue;
btnNext.Text = hdfTabIndex.Value + "Tab";
}
Hope it helps.
--Amit