It takes at least two, if you want to align both position and rotation. With a single fiducial (note spelling) mark you can only match position, but will not know if the PCB (or the films) are slightly rotated. So it would probably be good to have two placed in diagonal corners to maximize the distance between them.

But the actual needs depends on the manufacturer, so you should check. In some situations, you can have zero fiducials on your PCB as long as there are fiducial marks on the full panel. Some may for example require that the panel have one marker in each corner (called global panel fiducials).

Another thing is that when you have very fine-pitch components, there may be a need to have special local fiducials local to these critical components, for maximum mounting precision.