I think that for my exams I revised four 'D' courses and three 'C' courses. I got a high 2.1 out of that, which could have been a first if my revision had been better (i.e. if I'd known more!)
It's not clear-cut at all, though - it obviously depends on how thoroughly you know the courses that you decide to revise. It's obviously better to know a few courses really well than lots of courses partially, though.