You can make up to 3 diaries at a time. You may open a new diary after one of your first diaries is finished (must have at least 30 entries to finish) but you can only have 3 open at a time. You may also cut a diary if it is too hard to complete, and you can open a new one.
You can write from the perspective of a Pokémon (Diaries of the Eternal) or a human (Diaries of the Dying).