Let's say you could work out the optimal population size of a country. It would then be possible to work out the number of children required to maintain that population size over the long-term. Next you can work out how many children that is per person (rounded down and considering other factors like immigration) and set some rules.
Do you think it is necessary to control population size? Would you vote for this kind of system?
I haven't mentioned enforcement above as I see the question as more about the principle than the implementation.