I used to wonder about the Potentials who had been overlooked until they became too old to be called as well as those too young to fight effectively, but the possible answer might be something like this. In order to be considered a true Potential, a girl must be a certain minimum age (possibly when she reaches puberty), and loses her eligibility after she passes a certain age. That being the case, the chance for infant or geriatric Slayers would be eliminated. That would give Buffy's statement about eighteen hundred girls being found, but only five hundred "working" with them. The other eight hundred girls would be Slayers, but Slayers who had not yet come into their power. Hence, the "Slayers In Waiting," to differentiate them from the old Potentials, would receive their Calling once they reach a certain age, not when a present Slayer dies. All of these "S.I.W.s" would be either actually recruited at discovery or identified so that they can be recruited at a later date, closer to the time of their powers manifesting themselves. As for those too old to serve, some of them could be recruited to become Watchers os serve in some similar capacity, or simply allowed to live their lives as they saw fit.
If all of the above is the case, there might even be the possibility of a Slayer "retiring" when she reaches a certain age if she survives that long, but no one ever having lived that long, the Watchers simply aren;t aware of it.
