In another world I would've done 10 but I do 13 only because I don't have anything else to do (sport, tutor, work etc)
I see a lot of people in my grade with 10 units who don't actually utilise the free/extra study periods lol I'd stick to 12/13 because at least in that extra subject I'm learning something as opposed to studying/bludging in the school library where there's legit no monitoring or surveillance (not saying all of them bludge but of a lot of them do, however the snapchats do make me feel jealous tho lol)