Depends on the subject, though.
For example, actuarial is pretty small - the lecturer in charge marks all of every exam.
For first year accounting (back in the days when I did it), I think each lecturer got to mark his or her own part of the course.
For first year maths most of the staff/tutors get involved, because there's just too many to mark, and as Minai said, the guidelines are monitored carefully.