My java script seems to be not working

The javascript is suppose to check if the number of courses selected are atleast 4 and atmost 6. I'm populating my courses programatically. My javascript seems to be not working, can any one please help????

<script type="text/javascript">

         var atLeast=4
         var atMost=6

         function Validate() {

             var CHK = document.getElementById("<%=coursesList.ClientID%>");

             var checkbox = CHK.getElementsByTagName("input");

             var counter = 0;

             for (var i = 0; i < checkbox.length; i++) {

                 if (checkbox[i].checked) {
                     counter++;
                 }
             }

             if ((counter < atLeast) && (counter > atMost)) {
                 alert("Please select atleast " + atLeast + " courses" + " , but not more than " + atMost);
                 return false;
             }
             else {
                 return true;
             }
             
         }

     </script>

<p>
                NB!!! You can only register for a minimum of 4 courses and a maximum of 6 courses. Thank you
                <br />

                <asp:CheckBoxList ID="coursesList" runat="server" RepeatDirection="Horizontal" RepeatColumns="2">
                </asp:CheckBoxList>

                <asp:Label ID="alert2" runat="server" Text="Label" Visible="false"></asp:Label>

            </p>
            <asp:Button ID="page2" CssClass="button" runat="server" Text="Register" OnClientClick="return Validate()" OnClick="page2_Click" />