How many project staff worked as leaders or started working before '1989-04-24 23:51:54'?
SELECT count(*) FROM Project_Staff WHERE role_code  =  'leader' OR date_from  <  '1989-04-24 23:51:54'
SELECT count(*) FROM Project_Staff WHERE role_code  =  'leader' OR date_from  <  '1989-04-24 23:51:54'
[{'count(*)': 11}]