I decided to replay career missions, trying to double the required population, if it is possible to feed all those people.
This was my first try. It did not end up OK, because I kept the culture rating low, and that caused 2 problems: high unemployment, and angry gods. High unemployment prevented me from taxing people, and my whole treasure was spent. Angry gods (especially Venus) turned my people to crime as well.
Still, I was able to get to the targeted population. From then on, I used a different strategy, keeping favor low instead.