An applicant pool is usually judged on two measures: size, meaning how many people applied, and quality, meaning how many meet the essential requirements. A pool of 800 with 40 qualified applicants is a very different problem from a pool of 60 with 30. Pools are shaped by where the role is advertised, how the advert is written, pay, location and timing in the hiring calendar.
Pool composition also matters for fairness analysis, because selection rates at each stage are compared against the people who applied. A narrow pool limits who can be hired, however good the screening. Tracking qualified applicant rate by source shows which channels produce suitable people, and a pool that is large but weak often points to an advert that needs clearer requirements rather than more screening effort.