Someone

SELECT COUNT(1) AS avizate, m2.facturate, IFNULL(m3.retururi, 0) retururi, MONTH(orders.date_sent) month, YEAR(orders.date_sent) year
FROM orders
LEFT JOIN (
    SELECT COUNT(1) AS facturate, MONTH(date_purchased) month
    FROM orders
    GROUP BY month(date_purchased)
) m2 ON MONTH(orders.date_sent) = m2.month
LEFT JOIN (
    SELECT COUNT(1) AS retururi, MONTH(date_return) month
    FROM orders     
    GROUP BY month(date_return)
) m3 ON MONTH(orders.date_sent) = m3.month
GROUP BY month(orders.date_sent)