OnRowDataBound event of gridview you can add textbox value with gridview row count.
<asp:TemplateField HeaderText="SNo">
<ItemTemplate>
<asp:Label ID="lblSNo" runat="server"></asp:Label>
</ItemTemplate>
</asp:TemplateField>
protected void gv_OnRowDataBound(object sender, GridViewRowEventArgs e)
{
if(e.Row.RowType==DataControlRowType.DataRow)
{
((Label)e.Row.FindControl("lblSNo")).Text=Convert.ToString(Convert.ToInt32(txtsno.Text)+Convert.ToInt32(e.Row.RowIndex+1));
}
}