user: Question: How many years were recorded when world ranking was 21st?
Columns: ['Index (Year)', 'Author / Editor / Source', 'Year of publication', 'Countries sampled', 'World Ranking (1)', 'Ranking L.A. (2)']
Types: ['text', 'text', 'text', 'real', 'text', 'text']
Sample row: ['Global Peace (2009)', 'The Economist', '2008', 140.0, '21st', '2nd']
SQL:

assistant: SELECT COUNT("Index (Year)") FROM "Table" WHERE "World Ranking (1)" = '21st';