>As many as it takes to have a sustainable culture.

People were advocating for much the same thing in the '70s when Erlich's The Population Bomb came out, and thank God we ignored them. And yet a single hectare of land produces multiples of what it produced then, allowing enough food for everyone (though it doesn't always get to the people who need it, admittedly).

If I had to bet I'd say in 100 years we'll be producing even more food per person, so I consider any plan that would have people starving forced to limit reproduction to be off the table.