I’ve tested a new midrange Imac Pro (10 core, 16gig graphics, 64 gig memory) with extra monitors. Officially it supports 2 5K monitors or 4 4K ones. But I’ve found that it will not only run 2 5K LG monitors, but also two extra ones—a standard Dell 30” monitor, plus a 2012 Imac in Target Display mode. I don’t really need 5 screens, but it’s good to know it’s possible!