How many images have at least one object sample in the class of "man"?
have at least one object sample in the class of "man" refers to count(IMG_ID where OBJ_CLASS = 'man') > = 1
SELECT COUNT(T.IMG_ID) FROM ( SELECT T2.IMG_ID FROM OBJ_CLASSES AS T1 INNER JOIN IMG_OBJ AS T2 ON T1.OBJ_CLASS_ID = T2.OBJ_CLASS_ID WHERE T1.OBJ_CLASS = 'man' GROUP BY T2.IMG_ID ) T