I am finishing my basement. Walls and ceiling will be covered entirely. I need to install junction boxes behind the ceiling (up in the joists) or inside of the wall itself. From what I understand, the main consideration for this is that the junction boxes need to be accessible after the area is finished. For this I plan on using a wall panel to open/close access to this area.
What I have not been able to find information on, however, is how many you can put in proximity to each other. I have provided a picture here of a tentative install (they are nailed in place, but easily removable) of six junction boxes in an area between two wall studs. I will not be insulating this stud bay. I will fireblock the above area and ensure to my best ability that it is air tight with expanding foam and fireblock.
Is there any problem with this installation and does it satisfy NEC? (the yellow Romex here is NOT live by the way) 
Best Answer
There is no issue there with crowding too many junction boxes into the space, it's fine.
However, it looks like you have an issue securing the cables close to the boxes. No trouble for the top box on each side, but how will you route the cables to the middle and bottom?
The thing to do is add something in there to which you can secure the cables. There are many things you could use, but 2x4 scraps are probably readily available and won't cost you a dime.
You could cut three pieces of two by four to the width of the stud bay and nail it from stud to stud, standing on edge, at the back of the bay. One would go above the top boxes, one between the top boxes and middle boxes, and one between the middle boxes and the bottom boxes. (You might want to increase the space between boxes a bit just to make it easier to work around these.)
Now you can route your cables vertically down the middle of the bay, and staple them to each 2x4.