JSFiddle - React, Tailwind, and code Playground

HTML

<fieldset>
<legend>Packout</legend>
         <ul>
            <li>
            <label>How many units to packout?:</label>
            <select  id="quantity" name="quantity" class="validate[required]">
                    <option value="">Choose a quantity...</option>
                        <option value="1">1</option>
                        <option value="2">2</option>
                        <option value="3">3</option>
                        <option value="4">4</option>
                        <option value="5">5</option>
                        <option value="6">6</option>
                        <option value="7">7</option>
                        <option value="8">8</option>
                        <option value="9">9</option>
                        <option value="10">10</option>
                        <option value="11">11</option>
                        <option value="12">12</option>
            </select>
            </li>
        <li id="1" class="textboxes">
            <label>ISN 1:</label>
            <input  type="text" id="ISN1" name="ISN1" maxlength="12"  class="validate[required],validate[custom[hex]]"/></li>
            <li id="2" class="textboxes">
            <label>ISN 2:</label>
            <input  type="text" id="ISN2" name="ISN2" maxlength="12"  class="validate[required],validate[custom[hex]]"/></li>
            <li id="3" class="textboxes">
            <label>ISN 3:</label>
            <input  type="text" id="ISN3" name="ISN3" maxlength="12"  class="validate[required],validate[custom[hex]]"/></li>
            <li id="4" class="textboxes">
            <label>ISN 4:</label>
            <input  type="text" id="ISN4" name="ISN4" maxlength="12"  class="validate[required],validate[custom[hex]]"/></li>
            <li id="5" class="textboxes">
            <label>ISN 5:</label>
            <input  type="text" id="ISN5" name="ISN5" maxlength="12"  class="validate[required],validate[custom[hex]]"/></li>
            <li id="6"...

CSS

.textboxes {display:none}

JavaScript

var dropdown; 
$('#quantity').change(function() {
        $('.textboxes').hide();
        dropdown = $('#quantity').val(); 
    for (var i=1;i<=dropdown;i++){ 
        $('#'+i).show();
}
     $('#results').html(dropdown);
 });