Well, I know like six people trialling 15 units for Term4, Year 11 and atm they don't seem to be dying (and they are all normal--none accelerating in anything)...but it's up to you. Our school's said 12's the magical number, 13's pushing it, and 14 is just hell, but u know, they can't stop you. WHat do you wanna do at uni? If you're aiming for medicine or something sciency, eco might not be much use, and vice versa. I suppose you'll have to be good with your organisational skills to study 14 units, but you can always drop later in the year. If you're stuck, or tossing up between a few subjects as to which to drop, do the first Year 12 assessment for each subject and compare marks.
Hope that helped