How to properly relocate wiring on different circuits
You do not need a separate junction box for each line, nor do you need one per circuit. Unless you're strapped for space inside the box, one does just fine.

The National Electrical Code does say that the junction box has to be accessible "without removing any part of the building". So any junctions will need to be accessible through some sort of hole - normally covered by a simple faceplate. All the more reason to keep the number of boxes as small as possible.