There are currently two groups of mobs; "Hostile" and "Passive." Both have a limit of 30. If a mob goes over the total amount of a group limit, it will not spawn. If there are excess mobs from the "Hostile" group, the server removes them. The server does not remove excess mobs for hitting the "Passive" group limit. However, it will still remove certain types of mobs listed in the "Passive" group if they exceed an individual limit.