I can only speak for myself, but 2 was a good number for me. This amounts to somewhat less than a replacement rate. My wife and I had enough time and energy to give the kids what they needed, and still have some for ourselves at the end of the day. And if either of the kids needed extra resources or attention, we were able to do it without neglecting the other one.

I am not worried about a population decline, to be honest. Even disregarding AI, improvements in technology and food production mean we can leverage resources in a way that would seem like magic to the people alive when my grandparents were born. I would rather take care of the people we have in this world - the whole world, not just my country - than see more people born into slums and poverty.

Even if there is a cliff, I don't think it's an existential crisis. I say without irony, I believe the market will adjust. Wages will go up in jobs that are needed, and workers will have more leverage and more mobility, socially and geographically. It's hard for me to see that as a bad thing.

Even if you believe that technology will let us keep pushing the earth's carrying capacity indefinitely, to what end? It doesn't seem like anyone has a real plan for expanding beyond 8 billion that isn't just a promise that we'll figure it out when we get there. We aren't taking care of the people we have now. Never mind the ones yet to be born.

I don't want to live in Brave New World and I also don't want to live in The Dosadi Experiment. And I don't want to condemn the future people to live like that either. I know those are works of fiction, but both seem plausible (in the general sense) at this point.

(Edit: not Brave New World. I am thinking about a story where people lived in dense arcologies with tight surveillance and social control surrounded by robotic farms. Sorry I can't remember.)