.length

length ( ) Returns: Integerversion added: 1.0

Description: The number of elements in the jQuery object.

The number of elements currently matched. The .size() method will return the same value.

Examples:

Example: Count the divs. Click to add more. 

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>css demo</title>

  <style>

  body {
    cursor: pointer;
  }
  div {
    width: 50px;
    height: 30px;
    margin: 5px;
    float: left;
    background: green;
  }
  span {
    color: red;
  }

  </style>

  <script src="//code.jquery.com/jquery-1.10.2.js"></script>
</head>
<body>
  
<span></span>
  <div></div>
<script>

$( document.body )
  .click(function() {
    $( document.body ).append( $( "<div>" ) );
    var n = $( "div" ).length;
    $( "span" ).text( "There are " + n + " divs." +
      "Click to add more.");
  })
  // Trigger the click to start
  .trigger( "click" );

</script>

  
</body>
</html>

Demo: