For how many terms has current legislator Sherrod Brown served as a representative for district no.13?
district no. 13 refers to district = 13;
SELECT COUNT(*) FROM current AS T1 INNER JOIN `current-terms` AS T2 ON T1.bioguide_id = T2.bioguide WHERE T1.official_full_name = 'Sherrod Brown' AND T2.district = 13