Give the number of images containing the object sample of "suit".
number of images refers to IMG_ID; object sample of "suit" refers to OBJ_CLASS = 'suit'
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 = 'suit' GROUP BY T2.IMG_ID ) T