user: Question: How many season premiers have a rank of #21?
Columns: ['Season', 'Episodes', 'Time slot (EST)', 'Season premiere', 'Season finale', 'TV season', 'Rank', 'Viewers (in millions)']
Types: ['real', 'real', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: [1.0, 23.0, 'Wednesday 10pm/9c', 'September 22, 2004', 'May 18, 2005', '2004–2005', '#21', '13.59']
SQL:

assistant: SELECT COUNT("Season premiere") FROM "Table" WHERE "Rank" = '#21';