instruction: In Spring-Summer 2011 , how many professors taught BIT 644 ?
response: SELECT COUNT(DISTINCT instructor.instructor_id) FROM course, course_offering, instructor, offering_instructor, semester WHERE course.course_id = course_offering.course_id AND course.department = 'BIT' AND course.number = 644 AND offering_instructor.instructor_id = instructor.instructor_id AND offering_instructor.offering_id = course_offering.offering_id AND semester.semester = 'Spring-Summer' AND semester.semester_id = course_offering.semester AND semester.year = 2011