There are journal writers who, inspired by minimalists, will not own more paper journals than they need at one time. If they are writing, then that is the only (semi) blank journal in the house. This method keeps down unnecessary objects and clears the mind. The lack of visual noise can quell anxiety and create a peaceful space.
However, while trying to keep objects in the house down to a minimum, one might discover that a second blank book is useful for sustained journaling.

It has been this author’s experience that sometimes you might finish a journal before you finish a thought. It is best to have the next journal handy to continue jotting down the idea for a couple of reasons. One, in case you forget the idea before buying a new journal. Two, you want the inertia from writing in the finished journal to carry over into the next one.
There is something about a completed journal that feels final for some of us. Some of us do not want the finality of completing the book to lead to resting on our laurels and not picking up a journal again for a while . . . or ever again. Have an extra journal on hand to sustain the journaling habit.
When one has immediately opened a new journal to continue a thought from the previous book, one has broken in the new journal, played around in a fresh space, marked the territory. In this situation, one is likely to return to the book.
If a person is in a tug-of-war with a need to remain minimalist (to own only what you need) and a need to continue the inertia of journal writing by having an extra book around, then think of the extra book in the house as a need, because, in that case, it is a need.
Peace,
Deborah