If you want to display the *attached* image gallery, from another post, you can use:

    [gallery id="123"]

where the custom `id` attribute is the post ID.

If you want limit the number of items in a gallery, you can use:

    [gallery maxitems="50"]

where the custom `items` attribute is the maximum number of items to display.

This attribute is supported by the following plugin:

    <?php
    /**
     * Plugin Name:   The Gallery Shortcode - Maximum Number Of Items.
     * Description:   Support the 'maxitems' attribute in the gallery shortcode
     * Plugin URI:    http://wordpress.stackexchange.com/a/176937/26350
     * Plugin Author: Birgir Erlendsson (birgire)
     * Version:       0.0.2
     */

    add_action( 'wp', function()
    {
        if( class_exists( 'GalleryMaxItems' ) )
        {
         	$o = new GalleryMaxItems;
            $o->init();
        }
    } );
    
    class GalleryMaxItems
    {
        private $maxitems;
    
        public function init()
        {
            add_filter( 'post_gallery', array( $this, 'post_gallery' ), 10, 3 );
        }
    
        public function post_gallery( $output = '', $attr = array(), $instance = 0 )
        {
            if( isset( $attr['maxitems'] ) )
            {
           	    $this->maxitems = $attr['maxitems'];
         	    add_filter( 'query', array( $this, 'query' ) );
           	}
           	return $output;
        }
    
        public function query( $sql )
        {
     	    remove_filter( current_filter(), array( $this, __FUNCTION__ ) );

        	$limit	= sprintf( " LIMIT 0, %d ", $this->maxitems );
         	$regex	= '#LIMIT 0,\s\d+#';

         	if( false !== stripos( $sql, ' LIMIT ' ) )
                $sql = preg_replace( $regex, $limit, $sql );
       	     else
                $sql .= $limit; 

            return $sql;
        }
    
    } // end class