How to limit the content coming from wordpress shortcodes?

It’s better to 1st save the returned XML to a file and then loop back to unset.

<?php
  $curl = curl_init();

  curl_setopt_array($curl, array(
    CURLOPT_URL => "http://www.cpac.ca/tip-podcast/jwplayer.xml",
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_ENCODING => "",
    CURLOPT_MAXREDIRS => 10,
    CURLOPT_TIMEOUT => 30,
    CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
    CURLOPT_CUSTOMREQUEST => "GET",
    CURLOPT_HTTPHEADER => array(
      "cache-control: no-cache",
      "postman-token: 28025ee8-1e82-ce60-f6ae-f401118baa1c"
    ),
  ));

  $response = curl_exec($curl);
  $err = curl_error($curl);

  curl_close($curl);

  if ($err) {
    echo "cURL Error #:" . $err;
  } else {
    $fp = fopen(ABSPATH.'jwp.xml', 'w');
    fwrite($fp, $response);
    fclose($fp);
  }

  $xml = simplexml_load_file(ABSPATH.'jwp.xml');

  for($i = count($xml->channel->item); $i >= 2; $i--){
    unset($xml->channel->item[$i]);
  }

  $xml->saveXML(ABSPATH.'jwp.xml');

  ?>
  <script src="https://content.jwplatform.com/libraries/FZ8yNTef.js"></script>
  <center><div id="podcast" align="center"></div></center> 
  <script> 
  var PodcastplayerInstance = jwplayer("podcast"); 
  PodcastplayerInstance.setup({ 
    playlist: "<?php echo site_url(); ?>/jwp.xml", 
    androidhls: true, 
    preload: "auto", 
    height: 200, 
    width: 400,
    visualplaylist:false,
    stretching: "fill",
      "plugins": {
          "http://www.cpac.ca/tip-podcast/listy.js":{},
          'viral-2': {'oncomplete':'False','onpause':'False','functions':'All'}
      }
  });
  </script> 

In case you want the 2nd or 3rd element only, update the above code with the following

for($i = count($xml->channel->item); $i >= 3; $i--){
  unset($xml->channel->item[$i]);
}

for($i = 0; $i < count($xml->channel->item); $i++){
  unset($xml->channel->item[0]);
}