well it partly depends on ur school's format of the timetable. If there are not enough periods, the max you can do is 12. I think my school is like 13 if you are going to do it every single period of the week. Meaning no free periods (study periods), however, there are classes before and after school but I think it is only languages.
Someone in my school however did do 14 units but I think it has to do partly with him accelerating