I have a DS2422+ and even that only has 2 USB ports. But you can expand them with a USB-hub. I had 5 external drives connected over a single port without issues.
Synology’s limited list of “approved” HDDs.
You can use this script and put it as a scheudeled task "on boot". It will automatically add all your harddrives to the list of verified drives, disabling all warnings and errors with it.