How many businesses are opened for 24 hours?
opened for 24 hours refers to attribute_name = 'Open 24 Hours' AND attribute_value = 'true'
SELECT COUNT(T2.business_id) FROM Attributes AS T1 INNER JOIN Business_Attributes AS T2 ON T1.attribute_id = T2.attribute_id WHERE T2.attribute_value LIKE 'TRUE' AND T1.attribute_name LIKE 'Open 24 Hours'