Hi rtmitch224,
Post #2 here and the mattress topper guidelines it links to can help you use your sleeping experience and how much extra softness and pressure relief you are looking for to help you choose the type, thickness, and firmness of a topper that would have the highest odds of working well for you on the mattress you are using.
It would be unlikely that you would need more than about 3" and in many cases you may need less than this. Thicker is certainly not better and in most cases “just thick and soft enough” to provide the pressure relief you are looking for will be less risky in terms of support/alignment.
Phoenix