I always thought studying 3 subjects per day is good. That's just me because 3 subjects is an appropriate amount that you can digest pretty easily imho.
I always studied my best subject to study, my hardest subject to study and some normal subject i.e. maths, English and bio/business/cosmo.
I had about 3-4 hours of studying per day and probably more during hol or before exam.
I will initially give you some tips as to how to form your timetable or study plan.
First....consider when you study best. During that time, study your worst subject. And study your best subject when the environment is not that favourable because regarldess of your environment, u can still study that subject well.