I'd go with 7.5 billion. Just enough for me to rebuild the world as the immortal God King programmer. Hopefully I figure out another way to extend it during this period, as well as some other cool time travelling tech or something. The sky's the limit with that many years of my life!
Why would i want to keep on living on this world, when i'm about to get a chance to be reincarnated in another world?! Jokes aside, no one else's life is more valuable than your own, so how can it be bought with someone else's "cheap" life? My answer is 0.
They're all strangers to me. Yes, even to my own blood. At the end of the day when night falls on my lonely room, all men and women, young and old, rich and poor, weak and strong, wise or foolish, every one of them shall die. All but left is me and in this barren world of humans. Why am I not included? No human can live longer than 120+. I'm no longer considered human in a sense, I have reaped all of humanity in 24 hours to extend my own.
I'm not sure I would have the guts to make other people die just to live myself, yet I don't know if I would think the same under a life threatening situation
It seems kind of pointless to randomly sacrifice others to hang around so zero. If I could choose who by stating vague criteria then I would just so I can DeathNote some terrible people.
even if you add 2 times the total number of human population as the number of sacrifices i doubt humanity will extinct if you can put microbes, plankton, insect etc for the sacrifices.