It's only possible in African countries because of all the aid efforts, the fact of the matter is that you're not supposed to have 7-8 children in a family unit. It was only done in the past because the child mortality rate was higher, which medicinal advances fixed. 3-4 is an ideal growth rate, any higher causes the many of the same problems as immigration and lowers the quality of upbringing due to both parents being stretched thin between all 7-8 children, unless you think the state should have a hand in raising children which is a really bad idea right now for obvious reasons. This isn't purely the numbers game you've reduced it to, it's actually a lot more complex than that.
Consider first a small town, population of a few thousand proper, hard working white people consisting of a majority of average family units of husband, wife and two children. This is, on paper, the ideal replacement rate barring death by circumstance and is also a stable replacement rate for a workforce undergoing little to no growth. Throw in a third child for slow and steady growth and/or accounting for factors such as individuals withdrawing from society, because there are always some no matter what you do.
Next, consider the addition of a fourth child to every family unit, the time of the parents begins to be stretched thin between their responsibilities and upbringing quality begins to degrade a little as they cut corners. Society at large can sustain population growth like this by creating new jobs and exploiting new resources. In the extreme long term, it's not too bad and is what we've mostly been doing up to the modern age.
Continuing on with a fifth and maybe sixth child, both parents now must either pay little attention to their children as a whole, ignore some but not others or get assistance from the community to maintain the quality of upbringing. Anyone here who was raised in a large family will know what I'm talking about here. Society will need some pretty rapid and active expansion to sustain a population like this at an acceptable quality of life or sacrifice the quality of life for less strain on increasingly limited resources. There's less to go around for the rapidly expanding population.
Let's try not to think about what would happen in the long term if you had seven to eight children per family unit, let alone if you sustained that rate of growth. Hint hint, those idiots after the war tried this with the baby boomers and look where it landed us not even a century later.
The solution to a problem like this isn't to just make more people and hope for the best, that's worse than kicking the can down the road.
1, Immigration must be slowed, preferably halted altogether, and awareness spread about the unsustainability of this kind of population growth, as it strains limited and ever decreasing resources between greater and greater amounts of people.
2, Ethnic and cultural blocs need to be separated and reconsolidated, and not by force, this was one of Hitler's mistakes, he tried to accomplish this too fast and got his ass handed to him because of it.
3, The quality of upbringing needs to be addressed, 'modern' behaviour needs to be studied and dissected with the cause and effect chains of each and every facet of it taught in excruciating detail to the current and all further generations. Focus must be placed on the importance of the social contract, in which preceeding generations act in the interests, and to the benefit, of successive generations. Deviation from this must be ostracised at all costs as it is the Jew's primary means of undermining us.
4, Economic planning needs to take into account the availability of resources, population growth and other factors, usury needs to be abolished with debt being taken on and paid off purely at a 1:1 rate. A focus must be placed on producing and maintaining a surplus of goods to keep living costs low while avoiding oversaturation of the market. Unfortunately, this isn't compatible with a free market, I don't think.
5, This is the point where we can begin considering careful expansion, try it too quickly and you wind up with an ageing workforce and crumbling infrastructure after a generation or two. A given population's size must be extrapolated and/or incentivised and then plan infrastructure and manufacturing around that number.
This is why you can't just shackle a girl to a bed and use her as a baby factory until her baby factory don't work no more, it causes more damage in the long run than it fixes. Our population problems are somewhat of our own making because like the shortsighted idiots we are, we let the Jews talk us into abandoning the social contract. This is what we must work to fix and work to maintain. Just churning out more people willy nilly is going to create a generation of neglected kids whom are easy for outside forces to manipulate into their way of thinking. The worst part is, this has happened before.