You don't have to take any amount of exams - you have 4 attempts at each subject over 6 sessions, and it's up to you to arrange things to suit. However the best seems to be doing them all in 4 to leave yourself some slack. We recommend 4 - 5 - 5 in a particular order.
There used to be a trend to learn the answers, but the question banks are pretty much useless now for anything but revision - one student just reported that his Met exam contained 50% of questions that were not seen before. I myself am drumming up some more POF/Perf questions so there is a major overhaul going on.