If you are set on looking into higher math classes, a fair minority of Harvard freshmen, particularly those who come from stronger schools, come in having already taken some multivariable calc, linear algebra or have some working knowledge of the two - particularly those who elect to take the higher proof-based/honors-type math classes in freshman year (Math 23/25/55).
But this is by no means something necessary - according to the Crimson’s incoming Class of 2020 survey here:
http://features.thecrimson.com/2016/freshman-survey/academics/
The majority (>70%) had only taken up to Calc AB or BC.