//od Roman Meidl
add_shortcode( 'celkem-produktu', 'get_instock_products_count' );
function get_instock_products_count(){
global $wpdb;
// The SQL query
$result = $wpdb->get_col( "
SELECT COUNT(p.ID)
FROM {$wpdb->prefix}posts as p
INNER JOIN {$wpdb->prefix}postmeta as pm ON p.ID = pm.post_id
WHERE p.post_type LIKE '%product%'
AND p.post_status LIKE 'publish'
AND pm.meta_key LIKE '_stock_status'
AND pm.meta_value LIKE 'instock'
" );
return reset($result);
}