SELECT count( p.products_id ) AS total
FROM products_description pd, products p
LEFT JOIN manufacturers m ON p.manufacturers_id = m.manufacturers_id, products_to_categories p2c
LEFT JOIN specials s ON p.products_id = s.products_id
WHERE p.products_status = '1'
AND p.products_id = p2c.products_id
AND pd.products_id = p2c.products_id
AND pd.language_id = '1'
AND p2c.categories_id = '21' 

MySQL said:  
#1054 - Unknown column 'p.products_id' in 'on clause'