JSFiddle - React, Tailwind, and code Playground

HTML

<select name="cur_month" id="id_cur_month" multiple="multiple">
    <option value="">Month</option>
    <option value="jan">January</option>
    <option value="feb">February</option>
    <option value="mar">March</option>
    <option value="apr">April</option>
    <option value="may">May</option>
    <option value="jun">June</option>
    <option value="jul">July</option>
    <option value="aug">August</option>
    <option value="sep">September</option>
    <option value="oct">october</option>
    <option value="nov">November</option>
    <option value="dec">december</option>
</select>
<input id="id_cur_arrear_res" type="text" name="arrear" value="1" readonly="readonly" />

JavaScript

$("#id_cur_month").click(function () {
    var len = 0;
    $("#id_cur_arrear_res").children(":selected").each(function () {
        len += ($(this).val() ? 1 : 0);
    });
});