Is there a standard for how many screws should be used to secure a single floor cabinet to the wall, per linear foot of the width of the cabinet? E.g. if I only have one 2x4 available behind a 30" wide cabinet, is that not sufficient?
One 3" screw per available stud through the top rail is usually adequate unless specific conditions warrant more. For long cabinets, one every second stud is probably fine.
For your specific example, one screw is fine assuming additional anchoring to adjacent cabinets or end walls. If you find that things don't seem secure, add one low as well.