You seem to have solved the puzzle yourself.
Depending on the game and your hardware, you can* cram a surprising amount of clients in to a single PC. In Legion I was running 15 accounts on a single PC with a 6-core CPU and I considered it playable in the open world and LFR. There were definitely trade-offs that needed to be made to make it happen though, and how much of that you're comfortable with varies from person to person. For example in Shadowlands I ran 1 client per PC with trash office PCs with no dedicated GPUs and those ran perfectly fine for "background" clients.
There really is no real chart that would show you what amount of clients requires what, since there are a substantial amount of variables to consider. What hardware do you have? 3 at the lowest settings maxing out your setup sounds odd to me unless your system is very dated or bottlenecked in some regard.
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