Question

explain why there must be at least two lines on any given plane

Explanation:

A plane is determined by non - collinear points. Two points form a line. With three non - collinear points (minimum for a plane), two lines (each through two points) are formed.

Answer:

A plane is defined by at least three non - collinear points. Two points determine a line. So, to define a plane (using the point - line - plane postulates), since a plane needs at least three non - collinear points, and two points make a line, there must be at least two lines (each line passing through two of those non - collinear points) on any given plane.