Hi princy,

Check this example. Now please take its reference and correct your code.

HTML

<asp:GridView ID="GridView1" HeaderStyle-BackColor="#3AC0F2" HeaderStyle-ForeColor="White"
    runat="server" AutoGenerateColumns="false">
    <Columns>
        <asp:TemplateField HeaderText="Gender">
            <ItemTemplate>
                <asp:DropDownList ID="genderDdl" runat="server">
                    <asp:ListItem Text="None" Value="None"></asp:ListItem>
                    <asp:ListItem Text="Male" Value="Male"></asp:ListItem>
                    <asp:ListItem Text="Female" Value="Female"></asp:ListItem>
                </asp:DropDownList>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="City">
            <ItemTemplate>
                <asp:DropDownList ID="cityDdl" runat="server">
                    <asp:ListItem Text="None" Value="None"></asp:ListItem>
                    <asp:ListItem Text="Mumbai" Value="Mumbai"></asp:ListItem>
                    <asp:ListItem Text="Pune" Value="Pune"></asp:ListItem>
                </asp:DropDownList>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Country">
            <ItemTemplate>
                <asp:DropDownList ID="countryDdl" runat="server">
                    <asp:ListItem Text="None" Value="None"></asp:ListItem>
                    <asp:ListItem Text="India" Value="India"></asp:ListItem>
                    <asp:ListItem Text="USA" Value="USA"></asp:ListItem>
                </asp:DropDownList>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Age">
            <ItemTemplate>
                <asp:TextBox runat="server" ID="age" CssClass="age" Text="1"></asp:TextBox>
            </ItemTemplate>
        </asp:TemplateField>
    </Columns>
</asp:GridView>
<br />
<button id="showRows" type="button" disabled="disabled">Add</button>
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<script type="text/javascript">
    $(function () {
        for (var i = 6; i <= 15; i++) {
            $("[id*=GridView1] tr:has(td):nth(" + i + ")").hide();
        }
        ValidateAgeTextBox();

        $('.age').on('blur', function () {
            ValidateAgeTextBox();
        });

        $('#showRows').on('click', function () {
            $("[id*=GridView1] tr:visible:has(td)").find('.age').attr('disabled', 'disabled');
            $("[id*=GridView1] tr:not(visible)").show();
            return false;
        });
    });

    function ValidateAgeTextBox() {
        var isEmpty = false;
        $("[id*=GridView1] tr:visible:has(td)").find('.age').each(function () {
            if ($(this).val() == '') {
                isEmpty = true;
            }
        });

        if (!isEmpty) {
            $('#showRows').removeAttr('disabled');
        } else {
            $('#showRows').attr('disabled', 'disabled');
        }
    }
</script>

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        System.Data.DataTable dt = new System.Data.DataTable();
        dt.Columns.Add("Id");
        for (int i = 1; i <= 15; i++)
        {
            dt.Rows.Add(i);
        }
        GridView1.DataSource = dt;
        GridView1.DataBind();
    }
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not IsPostBack Then
        Dim dt As System.Data.DataTable = New System.Data.DataTable()
        dt.Columns.Add("Id")
        For i As Integer = 1 To 15 Step 1
            dt.Rows.Add(i)
        Next
        GridView1.DataSource = dt
        GridView1.DataBind()
    End If
End Sub

Screenshot