I'm doing 10. But i technically have 12. Completed 2 in year 10, but the mark wasnt that great, so I'm not gonna be relying on that to get my UAI. xD
So, yeah. 12 is probably best, but only if you do good in everything. Having the mentality of, "Oh i can afford to not spend so much time on chemistry cause its just 2 backup units", you might as well drop it and do 10 and concentrate everything on those 10 units.