Edit in JSFiddle

$(document).ready(function () {
    $("input[name='tech']").change(function () {
        var maxAllowed = 2;
        var cnt = $("input[name='tech']:checked").length;
        if (cnt > maxAllowed) {
            $(this).prop("checked", "");
            alert('' + maxAllowed + 'つ以上は選べません！');
        }
    });
});

body {
    font-size : 12pt;
    padding :10px;
}

<p>2つ以上を選ぶとエラーが表示</p>
<br/>
<input type="checkbox" name="tech" value="List 01" /> List 01
<br/>
<input type="checkbox" name="tech" value="List 02" /> List 02
<br/>
<input type="checkbox" name="tech" value="List 03" /> List 03
<br/>
<input type="checkbox" name="tech" value="List 04" /> List 04
<br/>
<input type="checkbox" name="tech" value="List 05" /> List 05
<br/>