2020-11-10 : Let's say everyone controls lots of creatures. As Necrotc Hex resolves, first the player whose turn it is (likely you) chooses which six creatures they'll sacrifice, then each other player in turn order does the same, knowing the choices of players before them. Then all creatures are sacrificed simultaneously. Finally, you'll create the six tapped Zombies.

2020-11-10 : You'll create six tapped Zombies no matter how many creatures you sacrifice.