Which resources would be limited in the human population?
It depends on the area.
The most limited resource in an arid area would certainly be water. In modern times, this pertains most to developing countries because in wealthier countries, water is simply transported to dry areas. This is how Las Vegas can exist.
While it doesn't seem like a resource, space can be a big issue. In places like New York and many cities in Europe, they have to fit a growing population in a very restricted area, which means they have to partition the land very carefully. However, there are many places in the world where space is of no concern to a population.
Energy is another big one for some areas, especially some developing countries where their population is growing rapidly. Many times limits are placed on energy usage simply because power plants can't produce enough to power a large city. In many areas in Africa, for example, the population still burns wood as fuel. This may have worked to sustain a city of 1,000, but as that number reaches the millions, their forest goes away very quickly.