From experience: to 10 box: you need atleast a quality 6 core with 12 threads you can overclock to around 4.5 ghz or more and a decent motherboard that can supply more voltage than cheaper boards. You also need minimum 32 (each game can use up to 3 gb ram if you are using many addons) gb ram. Any newer grafixcard with 6 gb or more videoram. A good ssd. With this, assuming 1080 p on all windows and all slaves on low settings and renderscale at 50%...while you have semi good settings on your main and 100% renderscale.. you should be able to get a somewhat stable 60 fps on main and 30 fps on slaves. It will dip a bit in dalaran etc but elsewhere its mostly smooth gameplay.

Ideally though you should buy this pc for account 2 to 10 and get a seperate pc which only will be playing your main account. So you can play in 1080 p on settings 7 on your main So you can get pretty much buttersmooth visuals.