This rather fine answer reveals that most public betas have 3 moderators. Is there any reason why several sites have more and one site has fewer?
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I can answer it for Skeptics, we have 4 moderators because at some point we asked the SE team for a 4th pro-tem moderator and the SE team agreed with that. Skeptics is a site that needs a lot of moderator intervention compared to other SE 2.0 sites of a similar size. Additionally we knew that two moderators would be unavailable at the same time in the future, so we thought that a 4th position would be helpful.
Christianity.SE started with 5 moderators, I assume the SE team expected that a lot of moderator intervention would be required.
The amount of intervention by moderators varies a lot between sites. Many sites work perfectly fine with 3 or fewer moderators. Three is the default value for a new site, but the actual amount of moderators is decided on a case-by-case basis.
User Experience has 5 moderators because the the old SE 1.0 User Experience site was merged into beta (when it was still called User Interface), and the moderators from that site were made moderators of the SE 2.0 site.
(Programmers and Gaming (while not beta any more) have one more than the "standard" because the level of traffic on these sites is higher than the others and hence the level of moderation required is higher.)