1. I don't thinks there's a limitation on how many you can install, but they all need disk space, so it would be more of a question of disk space.
2. I don't think the version available on the repos can use USB. I think the one you can get at their site does.
3. Although you can always came here for questions, I think the
virtualbox forums has more detail information.
4. That's very simple, don't forget to erase the virtual hard drive!
5. It all depends on the OS. Most Linux distros will do OK, XP will do fine as well. I don't think Vista or Win7 would even boot with less than 1Gb.
I hope it helps.
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