The best answer to that line is what society will accept.

I mean, we already have a way to decide where that line is - supply and demand. In a perfect world people would decide not to have kids because its not financially possible based on the price due to shortages - like you say though that wouldn’t be the case.

With realistic considerations - your support from society ceases at two kids. If you want to have more no govt support.etc. That’s a vote killer as for some reason the governments responsible when you can’t feed your kids, but that’s the best way forward imo.