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.
To be honest, seeing the economic situations and increasing inflation all around the world, a couple would hardly prefer having more than 2 or 3 children since it becomes a challenge to sustain a proper livelihood for all of them as the expenses are always on a rise and someone who isn't rich and only earns a fixed salary that isn't much would think twice before planning another child. So before thinking about the economy of the country, one would probably think about their own condition and position in life.
It also depends on the country, there are countries where there is a lot of manual labor and they are always in need of workers in almost every sector. In such countries, having many children can be beneficial for the country as they won't need to import workers from foreign countries. But, if you live in a country where things are mostly automated, and there are more jobs than workers willing to work, a family doesn't need to have a lot of children.