How many games were published by Activision?
Activision refers to publisher_name = 'Activision';
SELECT COUNT(DISTINCT T3.id) FROM game_publisher AS T1 INNER JOIN publisher AS T2 ON T1.publisher_id = T2.id INNER JOIN game AS T3 ON T1.game_id = T3.id WHERE T2.publisher_name = 'Activision'