Among the products that get over at least 1 review, how many of them are from the mountain product line?
mountain product line refers to ProductLine = 'M'
SELECT SUM(CASE WHEN T2.ProductLine = 'M' THEN 1 ELSE 0 END) FROM ProductReview AS T1 INNER JOIN Product AS T2 ON T1.ProductID = T2.ProductID GROUP BY T1.ProductID HAVING COUNT(T1.ProductReviewID) > 1