Hi Thunderstorm, for me, I tried to study the subjects I learnt at school that day, the reason why I said I usually focus on one or two is because of the nature of my subjects which can be grouped together, for example, I usually study maths 2 unit with maths extension one, chemistry with physics, and when I wrote that exact quote, I had more in mind of holiday study rather than weeknight study, where I prefer to focus on one subject for self-study/revision during the school holidays. There really isn't a fixed number of subjects, the base line rule is that you focus more time on subjects that you are not as strong at, and that you find a timetable that is flexible enough but works for you. Study really is a trial and error thing, the advice I have given is not meant to be interpreted extremely literally, followed to the letter, but simply provides a model, a guideline by which you can modify and change to suit your own study needs. Hence I would say just trial and error, there are many different factors that prevents me from giving you a definitive answer, such as I don't know what subjects you study, what subjects you are good at, what your current study habits are, and because of this lack of information, this is the best answer I can give you based on the information I am given. Perhaps if you elaborated a bit further on how you structured your timetable in the past I might be able to assist you further.
Happy holidays to both of you