An account can have a maximum of 20 active characters on each server.
Note: The first 8 slots are free, the rest require a one time purchase per slot to unlock.
After the North American server merge, if you have more than 8 free characters, deleting one will not give you back that slot. For example, if you want to create a new free character and you have 10 free characters, you will have to delete 3 in order to create another character. However, you can still purchase a Premium Character Slot until you reach a maximum of 20 active characters.