What happens if i have 2 {Xathrid Necromancer}s, and while one is dying, then I sacrifice the one that isn't dying? How many zombies do I get? 3 or 4?
You would get 3. The first Necromancer hits the graveyard, it's ability that it hits the graveyard goes on the stack as well as the other Necromancer's since another human creature died. Then you sack the other Necromancer, then it's ability that it hit the graveyard goes on the stack while the other is already in the graveyard. So that means three of those abilities are on the stack. They all resolve, you got three zombies.