How many user's compliment in photo has medium in number?
user's compliment in photo refers to compliment_type = 'photo'; photo has medium in number refers to number_of_compliments = 'Medium'
SELECT COUNT(T2.user_id) FROM Compliments AS T1 INNER JOIN Users_Compliments AS T2 ON T1.compliment_id = T2.compliment_id WHERE T1.compliment_type = 'photos' AND T2.number_of_compliments = 'Medium'