How to get override the WP Admin Syndication feed limit for a custom feed?

I need to override the setting in the WP Admin for the number of recent posts in the Syndication settings under General -> Admin.

I'm using the following code, which gets all posts, but I don't need that many. Can someone give me an example of how to retrieve 50 posts? This should only affect this one feed and not all the others.

function no_limits_for_feed( $limits ) {
    return '';
}

add_filter( 'post_limits', 'no_limits_for_feed' );

UPDATE: Here's an example of the code I am using, which is from the Feed JSON WordPress plug-in:

class feed_json {
    function feed_json() {
        global $wp_rewrite;

        add_action('init', array(&$this, 'add_feed_json'));
        add_action('do_feed_json', array(&$this, 'do_feed_json'), 10, 1);
        add_filter('template_include', array(&$this, 'template_json'));
        add_filter('query_vars', array(&$this, 'add_query_vars'));

        $plugin_basename = plugin_basename(__FILE__);
        add_action('activate_' . $plugin_basename, array(&$this, 'add_feed_json_once'));
        add_action('deactivate_' . $plugin_basename, array(&$this, 'remove_feed_json'));
    }

    function add_feed_json_once() {
        global $wp_rewrite;
        $this->add_feed_json();
        $wp_rewrite->flush_rules();
    }

    function remove_feed_json() {
        global $wp_rewrite;
        $feeds = array();
        foreach ( $wp_rewrite->feeds as $feed ) {
            if ( $feed !== 'json' ) {
                $feeds[] = $feed;
            }
        }
        $wp_rewrite->feeds = $feeds;
        $wp_rewrite->flush_rules();
    }

    function add_query_vars($qvars) {
      $qvars[] = 'callback';
      $qv开发者_开发技巧ars[] = 'limit';
      return $qvars;
    }

    function add_feed_json() {
        add_feed('json', array(&$this, 'do_feed_json'));
    }

    function do_feed_json() {
        load_template($this->template_json(dirname(__FILE__) . '/feed-json-template.php'));
    }

    function template_json( $template ) {
        $template_file = false;
        if (get_query_var('feed') === 'json') {
            $template_file = '/feed-json.php';
            if (function_exists('get_stylesheet_directory') && file_exists(get_stylesheet_directory() . $template_file)) {
                $template_file = get_stylesheet_directory() . $template_file;
            } elseif (function_exists('get_template_directory') && file_exists(get_template_directory() . $template_file)) {
                $template_file = get_template_directory() . $template_file;
            } elseif (file_exists(dirname(__FILE__) . '/feed-json-template.php')) {
                $template_file = dirname(__FILE__) . '/feed-json-template.php';
            } else {
                $template_file = false;
            }
        }

        return (
            $template_file !== false
            ? $template_file
            : $template
            );
    }
}
new feed_json();

I thought you could hook into pre_get_posts, but if it is a feed the posts_per_page value gets overwritten later. So just hook into post_limits, check for your feed, and return a different LIMIT part if needed.

add_filter( 'post_limits', 'so6230475_post_limits', 10, 2 );
function so6230475_post_limits( $limits, &$wp_query )
{
    if ( $wp_query->is_feed( 'json' ) ) {
        $limits = 'LIMIT 50';
    }
    return $limits;
}