How to limit number of files uploaded with plupload integrtion module?
The way I limited it to single file, was to create js file and attach it to the form. In that file, I bound a function to QueueChanged, to remove excess files and hide "Add file" button:     

(function($) {

Drupal.plupload = Drupal.plupload || {};

/**
 * Attaches the Plupload behavior to each Plupload form element.
 */
Drupal.behaviors.download_plupload = {
  attach: function (context, settings) {
    $(".plupload-element", context).once('download-plupload-init', function () {
      var add_button = $(this).find('.plupload_button.plupload_add');
      var uploader = $(this).pluploadQueue();
      uploader.bind('QueueChanged', function() {
        var local_uploader_element = this;
        // Remove files exceeding the cardinality setting.
        if (local_uploader_element.files.length &gt;= 1) {
          var i = 0;
          for (i=0;i&lt;local_uploader_element.files.length;i++) {
            if (i &gt;= 1) {
              $(this).removeFile(uploader.files[i]);
            }
          }
          add_button.hide();
        }
        else {
          add_button.show();
        }
      });
    });
  }
};

})(jQuery);


This is a simplified version of js found in FileField Sources Plupload, trimmed, without Field API specific parts, and made to work on all plupload elements.