Most people lack the intestinal fortitude to try a stunt like this, but late one evening we decided to gather up every FireWire device in the building and see just how many we could connect to a single port. We had a stack of 18 hard drives, an iMac in FireWire Target Disk mode, a CD-R drive, and a camera hooked up before the system choked.
In setting up this monstrosity, we discovered that one drive doesn't always equal one device. We found that some hard drives - the VST, for instance - take up two device IDs. And each device requires its own external power brick.
The moral of the story? You may never need to approach FireWire's theoretical limit, but it is possible to wind up with 520GB of storage - and one hell of a mess to clean up.