Anything I write is pure speculation, but here goes:

If by "People of Gondor" you mean the descendants of the Dunedain, then I would estimate the average number of children as 2 per couple. Being long-lived, the Dunedain tended to marry late and have few children. Since their numbers seem to have steadily declined over the years, I assume that the deathrate exceeded the birthrate. If the deathrate significantly exceeded the birthrate, then perhaps the average number of children was closer to 2.5 - 3 per couple. In any case, I don't think you could describe the Dunedain as prolific breeders.