Alright so it's time to put this question to bed. I have an unpowered hub connected, now I shall connect 4 EHDs, 3 of them on the hub. I would connect more but I don't have anymore room for all the wall warts and I don't have a spare extension cord handy. Maybe I'll test 5+ in the future, but I suspect that if 4 works then 5+ will work.
Okay, it's connected. Only two show up on the DVR menu, however you can access and transfer recordings to/from #3 and #4. Looks like you can only playback recordings on #1 and #2. Order is determined by what order that they are plugged in and how long it takes to initialize e.g. an empty drive will initialize nearly instantly whereas a full drive will take a couple of minutes. Another thing I noticed, you no longer have to reboot to format an EHD on Hopper.
It looks like there may not be enough room on the transfer for 5+ EHDs, but I could be wrong on that though. Maybe a scrollbar would pop up.
EDIT: I am also able to transfer content to/from all 4 at once.