user: Question: How many years did BBC One rank 20th?
Columns: ['Year', 'Broadcast date', 'BBC One total viewing', 'BBC One Rank', 'BBC Two total viewing', 'BBC Two Rank']
Types: ['real', 'text', 'real', 'text', 'text', 'text']
Sample row: [1999.0, '12 March', 6830000.0, '20th', '3,130,000', '13th']
SQL:

assistant: SELECT COUNT("Year") FROM "Table" WHERE "BBC One Rank" = '20th';