The value of the acceleration due to gravity (g) can be derived from the gravitational force of attraction between the Earth and any body and the acceleration of a body when a force acts on it. The Earth exerts a force equal to F = G*Me*m/r^2 on any object with a mass m and at a distance r from the center of the Earth. The value by which any object is accelerated is equal to G*Me/r^2 where G is a universal constant.
The increase in the population of humans on the Earth does not change the value of g as each person that is born is created from substances derived from the Earth. The organic matter, minerals, water, etc that the human body is made of has been obtained from the Earth. A human baby does not bring with it matter from space to the Earth. As a result the total mass of the Earth Me will not change no matter how large the human population increases to.
The value of g on the surface can change only by:
1. If there is a change in the radius of the earth on surface
2. If their is change in total mass of the earth
As none of the two change with the change in population, therefore the value of g remains unchanged.