If you are doing a straight Commerce course (not combined) this means you have 144 UOC (units of credit). Incase you are wondering why - and for disambiguitive purposes - this is because it is a 3 year course, with 2 semesters a year and 24 units of credit a semester.
Of these 144 UOC, 36 of them are 'core' subjects that you have to do. This translates into 6 core subjects, which are split between the two semesters in your first year (3 each semester). This translates to 108 UOC left for working towards your major/s. The question of how long you can 'mess' around with your free UOC is dependent on how many majors you are planning to take. If it is one major, you have to have 1 major and 1 minor, with the major consisting of 48 UOC and the minor 24 UOC, leaving you with 24 units of credit to 'play' with - with the further remaining 12 UOC (thus the aforementioned 108 UOC total) being mandatory general education units.
If you are planning on double majoring, this means 42 UOC per major (total of 84 units) and 12 units of credit to 'play' with, and then a further 12 UOC being the mandatory general education units (thus totalling 108 as before).
So, in summary, if you are planning on doing 1 major, you can mess around with 4 subjects, whereas if you are doing 2 majors, you can only mess around with 2 subjects. The important thing to note is that in order to complete a major you must follow a sequence of study, and some courses (aka. subjects) require the completion of prerequisite subjects.
What does this mean in layman's terms? Essentially, choose your major before the end of Year 1, regardless of whether or not your single or double majoring. This is advisable because if you do not, you may find that although you have the hypothetical free UOC to do a single or double major, you cannot do the courses required to fulfill the major/s in time, because you cannot, for instance, do a level 3 accounting subject at the same time as its prerequisite level 2 accounting subject.
Feel free to ask me to clarify anything if I've been confusing.