As noted in the Functions Progression, the main focus is on functions of one variable:
Undergraduate mathematics may involve functions of more than one
variable. The area of a rectangle, for example, can be viewed as a
function of two variables: its width and length. But in high school
mathematics the study of functions focuses primarily on real-valued
functions of a single real variable, which is to say that both the
input and output values are real numbers. One exception is in high
school geometry, where geometric transformations are considered to be
functions. For example, a translation T, which moves the plane 3 units to the
right and 2 units up might be represented by T: (x,y) –> (x+3,y+2).