The default number of posts for WP_Query is ten posts per page.
Try adding 'posts_per_page' => -1
to your $wpec_args
:
$wpec_args = array(
'post_status' => 'publish',
'post_type' => 'wpsc-product',
'posts_per_page' => -1,
'wpsc_product_category' => $wpec_term_slug
);