How many businesses have a romantic ambiance?
romantic ambiance refers to attribute_name = 'ambience_romantic' 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 = 'true' AND T1.attribute_name = 'ambience_romantic'