Living forever is somewhat an exception. But even if people live forever there will still be a hard bound. Imagine a population of 8 people (4 couples). With on

Living forever is somewhat an exception. But even if people live forever there will still be a hard bound. Imagine a population of 8 people (4 couples). With one child per couple the first 4 couples will have 4 kids. These 4 kids will have on child per couple giving 2 kids. The 2 kids make a single couple which can have one more kid. The last kid cannot make a couple with anyone without someone having had more than one kid. So starting with 16 immortal people having immortal kids you cannot get a population above 15. (8+4+2+1=15) So for a stable population in the long run couples need to be having on average around 2 kids each (assuming they are not immortal, if everyone is immortal then no kids are required for a stable population). Of course if we take "couple" very literally then perhaps we might allow people to have as many kids as they chose provided they have each with a different partner (to form a new "couple"). This seems like rule bending though. Without rule bending one kid per couple just couldn't work in the long run.