I always felt that the earth was getting too crowded and hoped we could slow growth at least. I feel 10 billion could be a comfortable number but we may pass that before this century is out. I think 1 child may be harsh for certain areas of the planet that already are experiencing birth rate declines like Europe and North America. Maybe base it off the existing population per land area or GDP per capita something like that. If you have room for more people and you can feed and employ more people you can have more people.