Displaying ASP GridView row index number

You can achieve this through two way:

Method 1

Step 1. Add this your GridView:

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" DataKeyNames="id"
            DataSourceID="SqlDataSource1" OnRowCreated="GridView1_RowCreated" >

            <asp:TemplateField HeaderText="RowId">
            <ItemTemplate><asp:Label runat="server" />                      

Step 2: Add following GridView Event code snippet to your code – behind file (.cs)

protected void GridView1_RowCreated(object sender, 
			System.Web.UI.WebControls.GridViewRowEventArgs e)
    if (e.Row.RowType == DataControlRowType.DataRow) {
        // Retrieve the Label from the first column.
        Label myLabel = (Label)e.Row.Cells[0].Controls[0]; 

        // Set Label text equal to the rowIndex +1
        myLabel.Text = e.Row.RowIndex.ToString() + 1; 

Method 2
The another way to do it is this one in the Columns section of your GridView control:
<asp:GridView ID="GridView1" runat="server" AllowPaging="True" AllowSorting="True"
        AutoGenerateColumns="False" DataSourceID="SqlDataSource1" PageSize="5">
                    <%# Container.DataItemIndex + 1 %>
            <asp:BoundField DataField="CategoryName" HeaderText="CategoryName" SortExpression="CategoryName" />
            <asp:BoundField DataField="Description" HeaderText="Description" SortExpression="Description" />