JSFiddle - React, Tailwind, and code Playground

HTML

<script src="//code.jquery.com/jquery-1.10.2.js"></script>
<input type="checkbox" name="check1" value="1">
<input type="checkbox" name="check1" value="2">
<input type="checkbox" name="check1" value="3">
<input type="checkbox" name="check1" value="4">
<input type="checkbox" name="check1" value="5">    
<div id="checkcount"></div>

JavaScript

var contador = function() {
  var n = $( "input:checked" ).length;
  $("#checkcount").text( n + (n === 1 ? " é" : " são") + " selecionados" );
};
contador(); 
$( "input[type=checkbox]" ).on( "click", contador );