Among the salable products from the mountain product line, how many of them have the most reviews?
salable product refers to FinishedGoodsFlag = 1; 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 WHERE T2.FinishedGoodsFlag = 1 GROUP BY T1.ProductID ORDER BY COUNT(T1.ProductReviewID) DESC LIMIT 1