If you look at the graph where it says 0 minutes which would be on the origin and look up, it will be at 1,000 people which means 1,000 people were there before anything started.
Solutions to systems of equations are points at which the lines of the equations intersect. Knowing this, only equations representing the same line would have infinitely many solutions because they would intersect at every one of each other's points - they're the exact same line. Therefore, if they are not the same line, they would not intersect at each and every point, and would not have infinitely many solutions.