user: Question: What's the sum of the number of episodes that originally aired after 1991 with a series number smaller than 21 and a DVD Region 2 release date of 26 March 2012?
Columns: ['Series Number', 'Number of Episodes', 'Original Air Date', 'DVD Region 2 release date', 'DVD Region 1 release date']
Types: ['real', 'real', 'real', 'text', 'text']
Sample row: [1.0, 6.0, 1973.0, '2 September 2002', 'TBA']
SQL:

assistant: SELECT SUM("Number of Episodes") FROM "Table" WHERE "Original Air Date" > 1991.0 AND "Series Number" < 21.0 AND "DVD Region 2 release date" = '26 March 2012';