In 2015, how many complaints about Billing disputes were sent by clients in Portland?
in 2015 refers to Date received LIKE'2015%'; about Billing disputes refers to Issue = 'Billing disputes'; Portland refers to city = 'Portland'
SELECT COUNT(T1.client_id) FROM client AS T1 INNER JOIN events AS T2 ON T1.client_id = T2.Client_ID WHERE T1.city = 'Portland' AND T2.`Date received` LIKE '2015%' AND T2.Issue = 'Billing disputes'