I probably could have clarified: "N" stands for the number of lives you estimate this procedure could save above and beyond the "default". In other words, "Net future lives saved with body-transplant technology" minus "Net future lives saved without body-transplant technology"

An example would be, say that there are not any viable hosts for a cadaver's organs, so normally they would just have to cremate him which is +0 lives. But in this situation, they could transplant a head onto the body, which is +1 lives. And say that scenario plays out 50,000 times over the next however many years. So N=50,000.

Of course N will be much lower for you personally if you find that example (and other similar ones) unrealistic.