This setting controls the number of individuals in your population. If you set it to a number less than the existing population size, the end of the population (the less fit) will be deleted. If you set it to a greater number, random new individuals will be added.
A bigger population size means greater genetic variance, and therefore more powerful evolution per generation, but it also means that generations are slower to calculate. There is a balance between population size and number of generations run that you must find experimentally.