question: How many series originally aired before 1999 with more than 7 episodes and a DVD Region 1 release date of 16 april 2013?
answer: SELECT COUNT(series_number) FROM table_name_24 WHERE original_air_date < 1999 AND number_of_episodes > 7 AND dvd_region_1_release_date = "16 april 2013"
context: CREATE TABLE table_name_24 (series_number VARCHAR, dvd_region_1_release_date VARCHAR, original_air_date VARCHAR, number_of_episodes VARCHAR)