The amount of mobs that can spawn on your Minecraft server is limited by default. This can be a problem if you have a monster or animal farm on your server and wish to have a larger number of mobs on your farm.

You need to go into your server files (Files -> FTP File access on Multicraft) and open the file bukkit.yml.

The values that you need to edit are in this section:

spawn-limits:
  monsters: 70
  animals: 15
  water-animals: 5
  ambient: 15

Edit the values as you see fit, then save the file and restart the server.

Note: The more monsters or animals you have on the server the more of a performance impact you will have. This means that your server will start lagging if you have too many monsters or animals.

in Minecraft General