user: Question: how many date of birts are if the previous club is Chelsea
Columns: ['No.', 'Name', 'Place of Birth', 'Date of Birth', 'Previous Club', 'Date Joined', 'Fee', 'Contract Expiry']
Types: ['real', 'text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: [1.0, 'Ben Williams', 'Manchester', '27 August 1982', 'Carlisle United', '10 July 2009', '£60,000', 'May 2012']
SQL:

assistant: SELECT COUNT("Date of Birth") FROM "Table" WHERE "Previous Club" = 'Chelsea';