Hi telldurges,
Refer the below sample code.
HTML
<div>
    <asp:Label ID="lbl_alloted" Text="0" runat="server" Visible="false"></asp:Label>
    <div class="table table-responsive over-auto " id='example'>
        <table id="myTable_k" class="table table-striped table-bordered">
            <thead>
                <tr>
                    <th>
                        S.No.
                    </th>
                    <th>
                        <div class="checkbox checkbox-info m-0">
                            <asp:CheckBox ID="checkeddall" runat="server" />
                            <label>
                                Select
                            </label>
                        </div>
                    </th>
                    <th>
                        Substitution Pattern
                    </th>
                    <th>
                        Order No.
                    </th>
                </tr>
            </thead>
            <tbody>
                <asp:Repeater ID="rpt_SubsSetting" runat="server">
                    <ItemTemplate>
                        <tr id="ItemRow">
                            <td>
                                <%# Container.ItemIndex + 1 %>
                                <asp:Label ID="lblid" runat="server" Text='<%# Eval("Pat_Id") %>' Visible="false" />
                            </td>
                            <td>
                                <div class="checkbox checkbox-info m-0">
                                    <asp:CheckBox ID="chkSelect" onclick="CustomValidation(this)" Checked='<%# Eval("pat_check").ToString() == "True" %>'
                                        runat="server" CssClass="chkbox m-0" Text=" " />
                                </div>
                            </td>
                            <td>
                                <asp:Label ID="lbl_subjects" runat="server" Text='<%# Eval("Pat_Name") %>' />
                            </td>
                            <td>
                                <div class="col-md-6 p-0">
                                    <asp:HiddenField ID="hdnid" runat="server" />
                                    <asp:TextBox ID="TXT_periods" setcss="input-sm" texttype="Numeric" required="true"
                                        maxvalue="90" runat="server" Enabled="false" setfocus="true" setenable="true"
                                        Text='<%# Eval("Pat_Number") %>' />
                                </div>
                            </td>
                        </tr>
                    </ItemTemplate>
                </asp:Repeater>
            </tbody>
        </table>
    </div>
</div>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
<script type="text/javascript">
    var arr = new Array();
    function CustomValidation(ele) {
        var txt = $("input[id*='TXT_periods']");
        var chk = $("input[id*='chkSelect']");
        var checkedIndex = $(ele).closest('tr').index();
        if ($(ele).closest('tr').find($("input[id*='chkSelect']")).is(':checked')) {
            arr.push(checkedIndex);
        }
        else {
            arr = arr.filter(function (item) {
                return item !== checkedIndex
            });
        }
        for (var i = 0; i < arr.length; i++) {
            txt[arr[i]].value = i + 1;
            txt[arr[i]].disabled = false;
        }
        for (var i = 0; i < chk.length; i++) {
            if (!chk[i].checked) {
                txt[i].disabled = true;
                txt[i].value = '';
            }
        }
    }
</script>
C#
protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        System.Data.DataTable dt = new System.Data.DataTable();
        dt.Columns.AddRange(new System.Data.DataColumn[4] { 
                        new System.Data.DataColumn("Pat_Id", typeof(int)),
                        new System.Data.DataColumn("pat_check", typeof(bool)),
                        new System.Data.DataColumn("Pat_Name",typeof(string)),
                        new System.Data.DataColumn("Pat_Number",typeof(int)) });
        dt.Rows.Add(1, false, "John Hammond", 1);
        dt.Rows.Add(2, false, "Mudassar Khan", 3);
        dt.Rows.Add(3, false, "Suzanne Mathews", 2);
        dt.Rows.Add(4, false, "Robert Schidner", 4);
        rpt_SubsSetting.DataSource = dt;
        rpt_SubsSetting.DataBind();
    }
}
Screenshot
