How many samples of food object are there in image no.6?
samples of food object refers to OBJ_CLASS = 'food'; image no.6 refers to IMG_ID = 6
SELECT COUNT(T2.OBJ_SAMPLE_ID) FROM OBJ_CLASSES AS T1 INNER JOIN IMG_OBJ AS T2 ON T1.OBJ_CLASS_ID = T2.OBJ_CLASS_ID WHERE T2.IMG_ID = 6 AND T1.OBJ_CLASS = 'food'