I read through everything and I found nothing on this subject.
That’s because you can’t do that. The transition from one to two microphones is the biggie. That’s when you shop for a small mixer and you also find that cheap computer microphones don’t work any more.
Two mics to three is another serious step, but nothing like the first one. Three and up is just a matter of mixer size. Everything else stays the same.
“Can’t” is a slippery word. If you were in Linux or on a Mac, you can force two USB microphones to coexist as an Aggregate Device, but both become one show and you can’t split them. You can’t change relative volume during the show, either.
If you’re really good with a soldering station, you could probably rip two computer microphones apart and jam them both into one Mic-In on your computer. That would work badly because each mic (still mono) would arrive at a quarter volume or less.