Is there an effect both positive or negative to the county's economy when couples have plenty children?
In the past before the current economic era, around the 15th century and the beginning of the French industrial revolution, having many children was seen as fertility and good for both the family work force and country's also. The labour force was majorly manual and those who worked at the plantations were sort after as able bodied men or women.
But at the moment, what is the viability of having many children especially as it could reduce the per capital income.
In those days you are talking about, Education and white collar jobs weren't really a thing, i mean education was minimally available, white collar jobs were almost non existence, the major source of livelihood back then was farming, and a man in need to have a lot of children who will work on his farms, usually married a lot of wives.
But today civilization has taken over, there are now white collar jobs every where, and one of the most important criteria one must possess if he or she wants to work a white collar job, is that person must be educated, and your level of education most times will determine how high you get paid and also with less work.
Having a lot of children today, i would say can either be dangerous or healthy for the economy, it all depends on the couple having those children, if they couple are comfortable in both finance and mental ability to train those children well, in education and morals, those children can grow to become assets to the nation and the world, but if they are not properly trained, those children can grow to become a big problem to the nation and world at large, by becoming thieves, armed robber, drug and human traffickers and all manner of bad stuffs..
So it all depends on how those many children are brought up, that alone can tell whether they become assets to the economy or liability to the economy.