I have more than 20 devices connected, no problems. Using hubs though.
Indeed, a buddy of mine has at least 6 or so going at once for his 3D printing business, and we're considering a similar sort of strategy (w/ e.g. some box running Slic3r and Octoprint) so he ain't constantly running USB sticks back and forth.
That's... pretty much exactly the plan, yes. The printers we're using all (to my knowledge) run some version of Marlin w/ SD support, so we'd be doing this via the M28/M29 GCode commands over that same USB interface.
It ain't clear that OctoPrint supports this directly, but as long as it's able to shove the exact GCode we tell it to shove we should be able to make that happen. Since Marlin's supposed to wait until it gets an M29 command before it starts printing, it seems to me like low throughput would therefore be a non-issue (aside from, you know, the amount of time it'd take to push that data across, but that'd really only be an issue for the first time we're printing a new file).
That's ok. People have different needs. For me it is mostly many hard drives, exercise equipment, multiple cameras, some stuff I am testing and lots of smaller things.