I box 5 fine on this machine, at 3440 x 1440p (ultra-wide screen) and I get 30fps in Dalaran at peak times. You could probably 10 box on this hardware at 16:9 ratios like 1080p.

Mainboard: https://www.amazon.com/gp/product/B015JKNDJ0
CPU: https://www.amazon.com/gp/product/B012M8LXQW
RAM: https://www.amazon.com/gp/product/B01B4F3IJY
GPU: https://www.amazon.com/NVIDIA-GeForc.../dp/B078NZ9NYV

With an SSD drive of course.