Seriously! Just one hole?
If you want to know how many turns the shaft has made then yes only one.
How does the arduino know the location of the disk with just one slot?
It doesn't know the position of the shaft it just knows how many rotations it has made to the nearest rotation, If you want to know how many to the nearest quarter turn then have four holes. Encoders like this are incremental, that is to say they only tell you about the movement of the shaft but then I think this is all you need for your project.
If you want to know the angle of your shaft then you need something like an absolute shaft encoder. These are very expensive if they are optical. Hoverer there is a cheap magnetic one:-
But I don't think you need that at all.