That depends on your sag measurements. You have the size of the inserts to compare to the size of your sagging area. It’s simple math for those who follow the 3 steps and measure as instructed. Because everyone’s sagging situation is different, you can customize to meet your specific needs. You determine how thick of insert you need by your sag depth and then, estimate the overall size of your sagging area.
Some do use just one insert, some use 2, 3 and even 4 depending on their situation. You determine and control how much support is needed by measuring and then test your results with (1) No Sags insert to see how it responds with your type of mattress. This is a DIY solution, you simply fine tune your results to fit your particular situation.