Labor is Apportioned by Demand
For a village to be viable, it must at a minimum produce the goods necessary to the sustenance of the people. Beyond that, some measure of additional production contributes to the convenience and pleasure of its inhabitants. When all needs are filled, additional labor is unnecessary.
Once an economy has reached stability, it is sustained if each adult has one child to replace him. If a farmer raises several sons to the same work as their parent, there will be more farm labor than is needed. Some sons must not follow in the profession of their fathers, but learn a trade that is neglected, or otherwise leave the village to seek employment where their profession is in demand.
If a village is restricted to cultivating the same portion of land, it will grow to the population that can be sustained buy that land, and would not increase its population "in a thousand years." (EN: Based on the assumption that the same amount of food can be produced from the same amount of land, and innovation does not make the land more productive.)
The same may be said of any profession: the demand of goods in a given location has a limit, beyond which additional labor merely divides the aggregate demand among a greater number of producers: if a man whose work supports his family passes on his business to two sons, the business is divided among his sons, such that each has half as much as he needs. Likely one will surpass the other, and the latter must find for himself another profession.
By the same process of reasoning, the demand of a town, city, nation, or all of the world is finite for any given good. Just as two sons of a father, raised in the same profession, would not be able to sustain themselves, so would it be impractical if a second fishing village to appear where the needs of the people are sated by the one that exists.
Redundant example: if four tailors are enough to make all the clothing of a town, and a fifth arrives, any customers he attracts are at the expense of the other four, and each one will live more poorly.
There are instances in which some accident will cause a decrease in supply of a good from a given producer, which represents an opportunity for a new entry to a profession - but this merely replaces rather than augments the lost capacity.
Cantillon speaks to a particular concern of his time: the charity-schools in England and France that teach crafts. He finds this deeply concerning, because to teach ten thousand men to be carpenters merely gluts the profession and makes carpentry less profitable and sustainable as a profession.
(EN: This is a sound argument, but misses two key points that later economists will identify. First, each laborer is also a consumer, such that a farm village that doubles in size will also double the amount of food it consumers and thereby doubles the amount of farm-laborers it requires. Second, that demand of goods is theoretically finite but in practice extremely flexible: particularly for luxury items and durable goods, people can amass quite a lot.)