SELECT 
  c.category_id, 
  cd.category, 
  b.brand, 
  b.brand_id, 
  COUNT(DISTINCT p.product_id) AS product_count 
FROM 
  cscart_products p 
  JOIN cscart_products_categories pc ON pc.product_id = p.product_id 
  JOIN cscart_categories c ON pc.category_id = c.category_id 
  JOIN cscart_category_descriptions cd ON c.category_id = cd.category_id 
  JOIN cscart_brands b ON p.brand_id = b.brand_id 
WHERE 
  c.id_path LIKE '%712%' 
  AND p.status = 'A' 
GROUP BY 
  b.brand 
ORDER BY 
  product_count DESC 
LIMIT 
  16;

Query time 0.00132

Result

category_id category brand brand_id product_count
712 Вахтовые автобусы КАМАЗ 2461 2
712 Вахтовые автобусы ГАЗ 2312 1
712 Вахтовые автобусы УРАЛ 2950 1
712 Вахтовые автобусы УРАЛ СПЕЦ ТРАКТ 2951 1