Late to the party, but my answer is “whatever the max amount of RAM I could put on that MacBook when I bought it”.
Weirdly, it’s mostly an amortisation technique to lengthen the useful life of the machine and spread the cost out more.
With MacBooks , you pay a heck of a lot for the core components like the screen, casing, keyboard, trackpad etc. I don’t resent it, because they’re all of the highest quality and will last for many years.
But then, if you skimp on the memory, you artificially limit the useful life of the system (e.g 18GB - 3 years tops, IMO), and are forced to replace everything, even though there’s still plenty of life in them.
In fact, anyone who bought an M1 with minimum RAM back in 2021, would be looking at an upgrade where they pay again for the exact same screen, keyboard, trackpad etc, even though theirs are probably still great, just to get more performance.
OTOH, having gone all in and maxed the RAM, I’ve never once thought “I wish I had less memory”. The systems are an absolute joy to use, and allow me a degree of optionality that love. The first few years even offer a sort of “if I can’t do it on my dev machine, I probably can’t do it (economically) anywhere else” sanity check.
Finally, I hate switching machines, so the less often I have to do it, the more time I can spend on other things