Per usual with any game that has dozens and dozens of individual assets running around to track the work on the CPU escalates. However once the CPU is dealing with a reasonable number of assets the GPU pipeline begins processing more work raising its own bandwidth utilization.

Normally running one instance of a game you'd need a serious number of assets on screen to slow your modern CPU. Like a capital city raid or a deathball of 80+ toons phasing into sight. Multiboxing however puts us in the divisible circumstance where the CPU is no longer 100% effective.

Of course every machine in existence today has an upper limit on what it will handle with WoW. So forget about building one machine that will handle those rare scenarios without a FPS drop.

A better way to look at it is your intent. What will you be doing in the game? Where will your time be spent? Personally I am not in war mode ever in BFA thanks to their follow restrictions and I'm not usually near a capital. I don't raid and when I was playing BFA regularly my focuis was dungeons and transmogs. As a consequence I get by on a 4 core processor. Easily 5 boxing. If I go to 10 though I can see the FPS drop creeping up. Yes, even running old raids. Capital cities are a forget about it circumstance.

I'll probably upgrade soon if I want to expand my interests with WoW. An aside I was fence sitting because the Valve Index released lately and I was on the first preorders but I ended up hating the experience so now I'm back on WoW. I don't really need to upgrade though if I am only 5 boxing.

TL;DR: any hardware from the last 5 years will do for 5 boxing.