JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.4/themes/ui-lightness/jquery-ui.css">
<div class='container'>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
    <div class='item'>item</div>
</div>

CSS

.inner-container {
    border: 2px dashed orange;
}

JavaScript

$(".container .item:nth-child(15n+1)").each(function(i){
    $(this).nextAll('.item').andSelf()
           .slice(0,15).wrapAll('<div class="inner-container"></div>');
});