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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 items="2"]

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 'items' attribute in the gallery shortcode
 * Plugin URI:    http://wordpress.stackexchange.com/a/176937/26350
 * Plugin Author: Birgir Erlendsson (birgire)
 * Version:       0.0.1
 */

add_action( 'init', function()
{
    if( class_exists( 'LimitGalleryItems' ) )
    {
        $o = new LimitGalleryItems;
        $o->init();
    }
} );

class LimitGalleryItems
{
    private $items;

    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['items'] ) )
        {
            $this->items = $attr['items'];
            add_filter( 'query', array( $this, 'query' ) );
        }
        return $output;
    }

    public function query( $sql )
    {
         remove_filter( current_filter(), array( $this, __FUNCTION__ ) );
         return preg_replace( '#LIMIT 0,\s\d+#', sprintf( "LIMIT 0, %d", $this->items ) , $sql );
    }

} // end class

Since the gallery shortcode is using get_posts() or get_children(), to fetch the gallery images, there's no direct way to filter those queries. Not even to change the suppress_filter attribute.

First I wrote a solution that chops down the number of images in the ids/include parameters, but that will not work for when the id attribute is set.

Then I wrote a solution that just duplicated the get_children() in the gallery to generate a new chopped list of ids, to include again in the gallery_shortcode() function via the post_gallery filter. But I didn't like this solution since then we have to query the images twice!

So therefore I choose the query filter to directly modify the LIMIT of the generated SQL query. This should work no matter what combination of shortcode attributes you use.