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.
http://www.businessinsider.com/valery-spiridonov-head-transplant-2015-4
Don't really have much to add beyond this. It's pretty awesome stuff though. The fact that this is even within the realm of possibility makes the argument for cryonics that much stronger.