If multiple Zombies enter the battlefield under your control at the same time, Geralf’s last ability will trigger for each of those Zombies. Each of those triggered abilities will see each of those other Zombies that entered the battlefield plus any other Zombies that entered the battlefield under your control so far this turn.

Geralf’s first ability will consider spells you cast before it was on the battlefield. For example, if Geralf is the first spell you cast during your turn, each subsequent spell you cast that turn after it’s on the battlefield will cause its first ability to trigger.