<select id="mylist"><option>Test</option>
<option>Test</option>
<option>Test</option>
<option>Test</option>
</select>

Event.observe('mylist', 'change', checkList);

function checkList(e)
{
    var count = 0;
    var listObj = this;
    // Count selected items
    for (i=0; i < listObj.options.length; i++) {
        if (listObj.options[i].selected) {
            count++;
        }
    }

    // If the count is over, tell me the last selected item
    if (count > 10) {
        var e = e || window.event;
        var tgt = e.target || e.srcElement;
        alert(tgt.nodeName + ' Index:' + tgt.index);
    }
}

function checkList(e)
{
    var count = 0;
    var listObj = this;
    // Count selected items
    for (i=0; i < listObj.options.length; i++) {
        if (listObj.options[i].selected) {
           // If the count is over, tell me the last selected item
           if (++count > 10) {
               alert('Index:' + i);
               return;
           }
        }
    }

}

$$('select#mylist option[selected]:last')