Keeper Limits limit the number of Keepers each player can have in play.
Whenever a new Keeper Limit is played, place it face-up anywhere on the table. If there was already a Keeper Limit in play, discard it and replace it with this new one.
It's rules change takes effect immediately, so if it is currently not your turn, you have to adhere to the Keeper Limit. In other words, players would have to discard Keepers to comply to the new limit you just played.
For example: If you played a Keeper Limit 2 card, all other players would immediately discard Keepers they have in play, until they only have 2 left in play. Then at the end of you turn, you would would do the same.