I get this question often enough that I should write a post on it.
1) You need to keep in mind that not all of the exams are offered during each exam session. So make sure that you plan well so that you don't have to wait long to complete all your exams. All of the core subjects should be offered in every exam sitting.
2) How many should I write on my first try. Some people like to test the waters by writing only one exam on the first sitting. If you're in that situation then I recommend writing Constitutionial Law as your first exam. It gives you a good foundation for all the other subjects. I recommend that you write as many exams as you possibly can in one sitting. The number of exams you can write will depend on your personal circumstances. Some may be able to write 5 and some people may only have time for 1.
3) With respect to the core subjects, Foundations of Canadian Law overlaps with Constitutional Law, Administrative Law, & and tiny bit of Cirminal Law.
The Foundations exam tends to be very tricky and highly unpredictable and because of this I recommend that Foundations be taken with Administrative law and Constitution law in the same exam sitting. If you feel that you cannot handle three exams in one go, then cut out Foundations and leave it for another session. Why? Because you'll know the other two topics well enough to answer the Foundations exam. It is difficult to superficially learn the whole of Administrative Law based on two cases in the Foundations syllabus and then be expected to write a coherent exam answer which is worth 40-50% of your Foundations grade.
Criminal Law can be taken at anytime.
Professional Responsibility is one of the lighter exams and traditionally requires less preparation time than the other core subjects.
Those are my two cents. You'll find that others may have a different opinion on the matter.