How many persons in total have participated in 12 meter Mixed Sailing competitions?
12 meter Mixed Sailing competitions refer to event_name = 'Sailing Mixed 12 metres';
SELECT COUNT(T1.competitor_id) FROM competitor_event AS T1 INNER JOIN event AS T2 ON T1.event_id = T2.id INNER JOIN sport AS T3 ON T2.sport_id = T3.id WHERE T2.event_name = 'Sailing Mixed 12 metres'