A community with too many moderators may suffer from "over moderation" if too many users are moderating and not enough are doing whatever the community does. Additionally, the larger a moderation team, the more difficult it is to handle problems in a consistent manner.
A community with too few moderators though can easily get out of hand, making it difficult to bring things back under control later.
How can one estimate the number of moderators that are appropriate for a given community, based on its size, membership, etc? What metrics or other factors are most important in making this decision?