A login ID can contain up to 20 characters of letters, numbers or underscores, but NO spaces. If you attempt to enter an ID with spaces or more than 20 characters, the game may give you an error message when you try to save games or open saved games.
The game does not limit the number of login IDs you create. The only limit is the available file space within your designated folder. Change your folder settings to save new login IDs and games.