You can use the :selected selector to select them, then get the .length, like this:

var count = $("#mySelect :selected").length; 

Or, alternatively since a <select multiple> returns .val() as the array of selected values:

var count = $("#mySelect").val().length; 

You can use the :selected selector to select them, then get the .length, like this:

var count = $("#mySelect :selected").length;