From Arkham Horror Wiki
A token representing a monster
The monster limit is a cap on the number of monsters that can be in Arkham at one time. The limit is equal to the number of players in the game plus three.
If adding a monster to the board would bring the number of monsters above the monster limit, the monster is placed in the Outskirts area instead.
All monsters moving around Arkham City and in the Sky area count towards the monster limit. Monsters that appear at a location and are then removed do not count; nor do monsters in the Outskirts.
If the terror level reaches 10, Arkham is overrun and the monster limit is removed for the rest of the game.
Note: monsters on some other boards, such as Dunwich, do not count towards the monster limit.