Determining a station point for a perspective drawing
One or two vanishing points are placed on the horizon. If you're drawing with 3 vanishing points, the third will be above or below the horizon.



Just like the position of the horizon, the vanishing point(s) depend(s) on the perspective, and at what angle you are viewing the object you are drawing. In case of the one-point perspective, the upper and lower edge are parallel and horizontal, so any extra vanishing points would be at an infinite distance away.
In case of two-point perspective, vertical lines are parallel, so this is actually a simplified case of the three-point perspective.