5-10 is a bit of wide spectrum. 10 requires substantially more resources.

Additionally what you need depends a lot on what you wish to do while multiboxing and what kind of settings and FPS (on both background and foreground windows) you're willing to accept.

We could maybe throw together a build for that, but with so little knowledge it'll be hard to say whether or not it'll work for you. Especially since it's entirely plausible we'll see a not-yet-announced minimum hardware requirement increase for BFA.