Anecdotal and possibly incorrrect, but . . . I recall highlighitng a settler and seeing as many as 9 Mutfruits glowing. According to the info tips that display during loadscreens, "happier settlers are more productive" so it would seem to make sense that six food points is a baseline and it can go up from there.
Not saying I'm absolutely sure, but tentatively that is my observation. I'm honestly too busy with school work and just playing the game 'for fun' at the present time to systematically test this stuff using whatever console commands will work.
Edifying to hear that those scrapper benches can produce THAT much! 
Seems I need to focus a bit more on organizing my settlements.
So what stuff is 'exchanged' along supply routes? My understanding was that, I. any excess food from a settlement on a route would provide food to other settlements on the route, and also that II. any indivisible junk that is stored in any workshop (meaning raw materials that cannot be scrapped) is available to build with at any workshop on a trade route, however III. Weapons, apparel, aid, miscellaneous are not available and IV. (just guessing not sure) junk that is not already scrapped into indivisible components is also NOT available to build with at any location other than the one where it is stored. V. Also I am fairly certain that items that have been built and stored (structures, machines, furniture, whatever) at a Workshop are only available at that workshop and not any networked workshops.
With this in mind, I have developed a couple of habits:
1. 'making the rounds' to my settlements to collect anything in each workshop and take it back to my main base at Sanctuary
2. running trade routes through GreyGarden, so a typical network would look like: Sanctuary -> Red Rocket -> GreyGarden -> Oberland / GreyGarden -> Covenant -> County Cross -> Nordhagen.
The idea of point #2 being: exchange b/w settlements depends on connection to the network via any number of intermediate connections, and does not depend on a direct connection between two settlements (e.g., a connection b/w Sanctuary and Nordhagen is not necessary to supply Nordhagen as long as some intermediary connections exist) and keeping the connections limited for most settlements reduces overhead.