How does USB bandwidth work? Will multiple devices share the 480mbps limit for USB 2.0?

Some mainboards have multiple USB controllers, especially with USB3 this seems to be a relatively common configuration. In these cases you're sharing the specified bandwidth only among those ports that are connected to the same host controller. Some MoBos will have one controller dedicated to the front panel connectors and another for the rears. Others have different sorts of splitting.

afaik, yes. Not that it usually matters, 80-90MB is about the best you'll get out an internal hard drive normally. In the real world you can expect 40MB out of a usb 2.0 external drive.

But I think it's per hub, so normally your machine will actually have 3 x 2 port hubs or 2 x 3 port hubs, (or even maybe a 2 and a 4) so you likely wont find it's "max / number of drives"

Edit: for what it's worth though, i've done 8 external drives in raid and it sucked for performance, and raid 0 on 6 drives you're almost certain to lose your data.