24 For example, mission stations recently founded (i.e., within the last ten or twenty years) will have to wait a long time before they can enjoy the effective assistance of a native clergy. To this problem is added the small number of missionary workers, who are scattered widely throughout an immense population where not infrequently non-Catholic ministers are also active; they therefore cannot possibly perform all the tasks they are called upon to undertake. In one district some forty priests are working very hard among a million natives of whom only 25,000 profess the Catholic faith. In another locality fifty priests are stationed in the midst of a population of 2,000,000 persons, where the care of 60,000 Catholics in the area alone requires almost full-time service.
Source: Fidei Donum (Vatican.va)