In order to study large-scale survey effects in observations, cosmologists make use of simulations. In these simulations, as many galaxies as we observe are generated, as if we create a brand new universe.
In a cosmological survey, not all galaxies are observed. This is due to various reasons, like weather, star sparkles, or survey design. Simulations have to account for the proper footprint of a survey, as well as holes that are masked out.