Sum up the number of business with "ambience_romantic" attribute.
"ambience_romantic" attribute 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 T1.attribute_name = 'ambience_romantic' AND T2.attribute_value = 'true'