1. I don't see why not. I'm assuming you're not sure on your last subject. You gotta enrol in everything before semester starts anyway, you can enrol in some subjects only and leave it half done to come back to do it later
2. Usually 4 subjects each semester, although it is possible for you to do this thing called overloading, which means doing 5 subjects in one semester and 3 in the other semester. This requires some kind of applications and a grade average high enough (70+ i think) to prove that you can handle the stress of 5 subjects in one semester, which probably means 20 hours of uni a week for commerce. People I know who have done it don't really recommend it that much, but this depends on the difficulties of the subjects you choose and whether they are avaliable only for certain semesters.