Hi there!
You will do the same number of units (subject) as if you were doing a single degree, which is four per semester (or 8 per year). From my experience, you might actually find that commerce units tend to have less contact hours than science units.
Of course, doing a double degree will extend your degree by a year, but that's about it

It might also be worth noting that you will not have any 'free' electives in a double degree i.e. you can't pick units that aren't commerce or science related (such as arts units), but you can still pick science and commerce electives. Also, in commerce, you'll have to do 6 core units, where you sample a unit from different schools within business and economics (such as accounting, business law, marketing). This means you'll have less room to pick commerce electives, compared to science (but there is still some choice).
Side note: If you're interested in doing arts units alongside your double degree, you can also do a Diploma, which extends your degree by a year again, but will allow you to do an arts major.
Hope this helps!
