David, I checked out your site, the problem was a conflict with your theme. The Flexible theme is setting the posts_per_page parameter globally, effecting all the queries on a page, like the slideshow, and not just the main query. Here's an example from epanel/custom_functions.php:
if ( $query->is_category ) {
$query->set( 'posts_per_page', et_get_option( $shortname . '_catnum_posts' ) );
}
I added a check to these functions using the is_main_query function so that it only effects the main query:
if ( $query->is_main_query() && $query->is_category ) {
$query->set( 'posts_per_page', et_get_option( $shortname . '_catnum_posts' ) );
}
Here's a full copy of the updated function for anyone who needs it.
The slideshow quantity option is now working correctly. I'm going to try to get in touch with the devs at Elegant Themes, see if I can get this fixed upstream.