How to add rows to gridview dynamically in asp.net

Sample Grid Code:

<asp:gridview id="Gridview1" runat="server" showfooter="true" autogeneratecolumns="false" xmlns:asp="#unknown">
         <columns>
         <asp:boundfield datafield="RowNumber" headertext="Row Number" />
         <asp:templatefield headertext="Header 1">
             <itemtemplate>
                 <asp:textbox id="TextBox1" runat="server"></asp:textbox>
             </itemtemplate>
         </asp:templatefield>
         <asp:templatefield headertext="Header 2">
             <itemtemplate>
                 <asp:textbox id="TextBox2" runat="server"></asp:textbox>
             </itemtemplate>
         </asp:templatefield>
         <asp:templatefield headertext="Header 3">
             <itemtemplate>
                  <asp:textbox id="TextBox3" runat="server"></asp:textbox>
             </itemtemplate>
             <footerstyle horizontalalign="Right" />
             <footertemplate>
              <asp:button id="ButtonAdd" runat="server" text="Add New Row">
                     onclick="ButtonAdd_Click" />
             </asp:button></footertemplate>
         </asp:templatefield>
         </columns>
     </asp:gridview>

Code to Set initial row

private void SetInitialRow()
  {
      DataTable dt = new DataTable();
      DataRow dr = null;
      dt.Columns.Add(new DataColumn("RowNumber", typeof(string)));
      dt.Columns.Add(new DataColumn("Column1", typeof(string)));
      dt.Columns.Add(new DataColumn("Column2", typeof(string)));
      dt.Columns.Add(new DataColumn("Column3", typeof(string)));
      dr = dt.NewRow();
      dr["RowNumber"] = 1;
      dr["Column1"] = string.Empty;
      dr["Column2"] = string.Empty;
      dr["Column3"] = string.Empty;
      dt.Rows.Add(dr);

      //Store the DataTable in ViewState
      ViewState["CurrentTable"] = dt;

      Gridview1.DataSource = dt;
      Gridview1.DataBind();
  }

To Add New Row

private void AddNewRowToGrid()
    {
        int j=0;
        while(j<convert.toint16(txtinputtextvalue.text))>
        {
        int rowIndex = 0;

        if (ViewState["CurrentTable"] != null)
        {
            DataTable dtCurrentTable = (DataTable)ViewState["CurrentTable"];
            DataRow drCurrentRow = null;
            if (dtCurrentTable.Rows.Count > 0)
            {
                for (int i = 1; i <= dtCurrentTable.Rows.Count; i++)
                {
                    //extract the TextBox values
                    TextBox box1 = (TextBox)Gridview1.Rows[rowIndex].Cells[1].FindControl("TextBox1");
                    TextBox box2 = (TextBox)Gridview1.Rows[rowIndex].Cells[2].FindControl("TextBox2");
                    TextBox box3 = (TextBox)Gridview1.Rows[rowIndex].Cells[3].FindControl("TextBox3");

                    drCurrentRow = dtCurrentTable.NewRow();
                    drCurrentRow["RowNumber"] = i + 1;

                    dtCurrentTable.Rows[i - 1]["Column1"] = box1.Text;
                    dtCurrentTable.Rows[i - 1]["Column2"] = box2.Text;
                    dtCurrentTable.Rows[i - 1]["Column3"] = box3.Text;

                    rowIndex++;
                }
                dtCurrentTable.Rows.Add(drCurrentRow);
                ViewState["CurrentTable"] = dtCurrentTable;

                Gridview1.DataSource = dtCurrentTable;
                Gridview1.DataBind();
            }
        }
        else
        {
            Response.Write("ViewState is null");
        }

        //Set Previous Data on Postbacks
        SetPreviousData();
            j++;
        }
        
    }

txtInputtextValue.text which is your text box for count of new rows

To set previous Data

private void SetPreviousData()
    {
        int rowIndex = 0;
        if (ViewState["CurrentTable"] != null)
        {
            DataTable dt = (DataTable)ViewState["CurrentTable"];
            if (dt.Rows.Count > 0)
            {
                for (int i = 0; i < dt.Rows.Count; i++)
                {
                    TextBox box1 = (TextBox)Gridview1.Rows[rowIndex].Cells[1].FindControl("TextBox1");
                    TextBox box2 = (TextBox)Gridview1.Rows[rowIndex].Cells[2].FindControl("TextBox2");
                    TextBox box3 = (TextBox)Gridview1.Rows[rowIndex].Cells[3].FindControl("TextBox3");

                    box1.Text = dt.Rows[i]["Column1"].ToString();
                    box2.Text = dt.Rows[i]["Column2"].ToString();
                    box3.Text = dt.Rows[i]["Column3"].ToString();

                    rowIndex++;
                }
            }
        }
    }

Call the initial value method in page load event

call add new row method at button click

Note: if you bind data from DB Call that data as initial value