This works without mods, it is all vanilla and it also works on a server.

Facts about the spawner

• This spawner spawns 10 Zombies at the time
  
• It takes between 5-10 ticks before it spawns 10 more
  
• 100 Nearby entities = 100 Zombies before it stop spawning new ones
  
• I made it to spawn the Zombies maximum 2 blocks from the cage
  
• And it begins spawning when a player is closer than 40 blocks. (you can not surprise this cage)
  

I can see three scenarios using such a massive spawn

1. Quest

Extremely valuable resource and a massive army of zombies to defeat.
I my case I made a cave that spawns upstream, and pushes out the zombies, the players have to fight throug water and zombies to reach and destroy the spawn cage

2. Adventure

A hidden location in the open so the players have time to run away.
In my case a villager thief is caught in a cage, and when the players goes there to see the strange construction, the zombies appear in a small cave entrance covered of vines

3. Trap

Pandoras box - There is a temple and in the middle there is a spawn cage disabled by 5 torches, an unlucky player might destroy one of the torches, and BANG 10 zombies appear directly. And then 10 more every 1/4 to 1/2 second until they are 100