Who sits down and determines that 8 is the correct number? Why not 4? Or 2? Or 16 or 32?

https://en.wikipedia.org/wiki/Lockstep_(computing)

Example: https://www.st.com/resource/en/datasheet/spc574k72e5.pdf

I other words, how over engineered is it.

Build for various Linux distros, and some of the BSDs. You'll encounter weird compile errors or edge cases that will pop up. Often times I've found that these will expose undefined behaviour or incorrect assumptions that you wouldn't notice if you were building for a single platform.