Depends what you like and your capability in each subject. I knew a bloke who did 20 units (though he did 4u maths when he was 12).
In my opinion, doing more than 12 units is a waste. Even doing 12 is unncessary IF you're confident in what your top 10 will be - I personally don't see much point spending time on an extra subject just because of the fear of the what-if-you-bomb-one-subject scenario.
Also, what are your plans for university? If you want to go into something quantitative/sciency, then it seems logical to keep the chem phys if your doing well in them. Conversely, if you want to go into eco/business, then it might be worthwhile keep those. This is, of course, assuming you perform equally well in all your subjects. If not, then just pick your strongest.
Shame that English can't be dropped!