How many samples of animal objects are there in image no.660?
samples of animal objects refer to OBJ_SAMPLE_ID where OBJ_CLASS = 'animal'; image no.660 refers to IMG_ID = 660;
SELECT COUNT(T1.IMG_ID) FROM IMG_OBJ AS T1 INNER JOIN OBJ_CLASSES AS T2 ON T1.OBJ_CLASS_ID = T2.OBJ_CLASS_ID WHERE T2.OBJ_CLASS = 'animal' AND T1.IMG_ID = 660