I'll try to be brief.
Ideally, you would survey the entire population and try to get as large a sample as possible when you are doing quantitative studies. However, finding the population and encouraging them to participate in research can be difficult. While you might have a very broad recruitment effort, you might see small numbers of people, particularly if the researchers have a very sensitive topic or if the population is difficult to reach. If you don't know how to find them, that can pose challenges to serving more people or getting a larger sample size.
We're hoping to overcome some of those challenges by working directly with the community to design that community and help us identify where we find people to participate in research.