Put all the checkboxes inside a DIV control and then do as the following sample
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
</head>
<body>
<form id="form1" runat="server">
<div id = "dvCheckBoxes">
<asp:CheckBox ID="CheckBox1" runat="server" />
<asp:CheckBox ID="CheckBox2" runat="server" />
<asp:CheckBox ID="CheckBox3" runat="server" />
<asp:CheckBox ID="CheckBox4" runat="server" />
<asp:CheckBox ID="CheckBox5" runat="server" />
</div>
<script type = "text/javascript">
var validCount = 1;
function Validate() {
var isValid = false;
var dvCheckBoxes = document.getElementById("dvCheckBoxes");
var chks = dvCheckBoxes.getElementsByTagName("input");
var cnt = 0;
for (var i = 0; i < chks.length; i++) {
if (chks[i].checked) {
cnt++;
}
}
if (cnt < validCount) {
alert("Please select atleast " + validCount + "!");
return false;
}
return true;
}
</script>
<asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="Submit" OnClientClick = "return Validate()" />
</form>
</body>
</html>
