lol 11units is enough back up if you have like 5 2u's and 1ext!
why would you do 15??? waste of time, and you'll burn out so quick.
i was doing 14units at the beginning of yr 12..just so i could say i was doing 14units LOL..dropped down to 13 (bye bye 4u maths), then to 12 (Bye bye ext 2 eng)...