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.
You have started a quite good topic on which only a few couples or the youngest want to talk because, in my place on the globe, people are having more than 5 to 10 kids, even though the inflation rate and debt rate are so high that on each life the debt is in the thousands of dollars. But still, having a poor life, people are not stopping as they have no knowledge of such things.
Your topic is good for such people, IMHO. Having more kids is not a bad thing if your government allows it, because in some countries governments discourage couples from having many kids in a short time span. All I have to say on this is that if you as a parent are going to have another kid, even if you have 2 or 3 already, you must think about his or her responsibilities, which you must fulfil. If you think you could give them a good life, then there is nothing bad about it. But at the start, every parent thinks they will provide a good life to their child, but many could not fulfil that promise because half of them are just lazy and didn't want to fulfil that commitment because they have no one to answer to for violating the promise.
But half of that is working firmly and hard for their child to provide them with a good life. Be responsible and be with them. That's all. And the effect will be accordingly: if you are not able to provide them with a good life, they are just a liability for the government, and if you have provided them with a good life, then they are not.
Now, there are different types of good life too; some think providing them with all the goods falls into the good life category, but no, providing them with the morale of living, the rights they have to follow, and the respect of society is what they have to think about. And etc. etc. are also necessary.