Should I interview one user and create one document per interview , or should I create just one journey with a high level big picture?
Are you referring to the Ux Journey Map? If so, you usually generate a persona after several interviews (several).
The important part is to set the criteria of the questions (without directing the answer) try to have a general view of the user (what does she/he do, how old is he/she, what does he/she like) and then study the behavior they have towards the matter in hand.
After some interviews (a lot of them) you'll start hearing some points in common or observing a pattern that repeats, those are the important things to set the persona. Once you have that persona figured out you can start doing the Ux Map.
For the questions of the interview (that is the most important part) I suggest you to have a quick view of projective psychology techniques and the topic map. View and review your questions! They must not suggest any kind of answer!