How many documents are there with the document type code "BK"?
SELECT count ( T2.Document_Date )  FROM Projects AS T1 JOIN Documents AS T2 ON T1.project_id   =   T2.project_id WHERE T2.Document_Type_Code  =   "BK"
documents with document type code \"BK\" refers to Document_Type_Code = 'BK'; count of documents refers to COUNT(T2.Document_Date);