With the html:
<select name="my_list" multiple="multiple">
<option value="one">1</option>
<option value="two">2</option>
<option value="three">3</option>
<option value="four">4</option>
<option value="five">5</option>
</select>One way to find the number of options selected:
$("select[name='my_list']>option:selected").length
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