How many times did Richard Durbin become a legislator in district 20?
Richard Durbin is an official_full_name;  official_full_name refers to first_name, last_name; district 20 refers to district = 20
SELECT SUM(CASE WHEN T2.district = 20 THEN 1 ELSE 0 END) AS count FROM current AS T1 INNER JOIN `current-terms` AS T2 ON T1.bioguide_id = T2.bioguide WHERE T1.first_name = 'Richard' AND T1.last_name = 'Durbin'