How many majors depends on the uni. I can't say for most unis, but USyd has 27 different science majors from biology and physics to more specialised fields like plant science and anatomy & histology. UNSW should have a similar number of science majors.
Again, I can't say for all unis but at USyd and UNSW you don't have to declare a major until the end of second year, where a major is 24 or more credit points in one field of study with the same course code (eg. PHYS for physics subjects, CHEM for chemistry, etc). But you should structure your second year carefully to make sure you can major in a subject.