How many images have "keyboard" as their object class?
images refer to IMG_ID; "keyboard" as object class refers to OBJ_CLASS = 'keyboard';
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 = 'keyboard'